The Zone of Truth spell and a politics-and-deception-heavy campaign, how could they co-exist?

Question

Some context: I'm a newbie DM running a campaign which is a remix of the Waterdeep: Dragon Heist adventure. It's a campaign full of politics, alliances, betrayals and deception. All the major powers in the city want the treasure and they are conspiring against each other and the party to win in The Big Game. And the party in order to get what they want would have to break the law, which in Waterdeep means a huge chance of finding yourself on a trial... Long story short, this campaign would be possible and exciting ONLY if all these folks would be able to lie to each other...

Here's what happened: one of the party members invested his precious spell learning resource on the Zone of Truth spell and he started to "turn it on" before EVERY social interaction with every significant NPC. During these interactions he would just be asking the NPC to say some things like "I haven't talked to Manshoon earlier this month" or "I'm not involved in any way with the kidnapping of Ranaer Neverember", etc. This obviously makes NPCs not able to lie about these plot-critical events...

Initially, I did so most of the NPCs would tell the party that the using of Zone of Truth is "impolite and inappropriate" for negotiations / business talk. So all of those major NPC refused to talk...

But then I thought: what about a potential trial or criminal investigation? Wouldn't all the trials then look like:

1. Bob's on the trial, accused of killing Alice.
2. The trusted 20-lvl judge casts Dispel Magic / Greater Restoration on Bob to ensure there are no Modify Memory shenanigans --> then he casts Zone of Truth --> then everyone in the court room can use Detect Magic to ensure that Bob is indeed affected by the ZoT spell's effect.
3. Bob then asked "please repeat the exact phrase: I did not kill Alice"
4. If Bob can't easily say that exact phrase - boom, he's guilty!
5. ...
6. PROFIT! Bob is going to jail, no lawyer salaries wasted! "Speedy Waterdeep Justice for You" 😂

So... I ended up nerfing the Zone of Truth spell for this campaign, almost banning it... It worked OK. Our campaign continues and the party is having a great time navigating thought the crazy politics of Waterdeep! But that guy who wanted to go crazy with the Zone of Truth spell left the campaign, so I still regret that decision, like I killed his player agency with that ruling 😥

From your DM-ing experience, what could be some smart ways to deal with the Zone of Truth spell in a campaign intended to have a lot of deception? Just ban this spell at the start? Or is there a better way?

Answer 1

Yes, Zone of Truth and heavy politics can coexist.

Here are some possible avenues to keep political intrigue alive despite the existence of Zone of Truth:

1. Plausible Deniability. Leaders just have to be somewhat careful with their wording, so they can always say "I told him to 'Take care of the problem.' I never asked him to break the law or hurt anyone!" if they're asked. To use a non-political example: Look into the Wells Fargo cross-selling scandal. Management didn't order anyone to break any laws, but set expectations that were impossible to meet without breaking the law and punished anyone who failed to meet them.

2. Evasive Answers. Zone of Truth states "Such a creature can be evasive in its answers as long as it remains within the boundaries of the truth." If pressed with follow up questions like "Didn't it cross your mind your orders might be interpreted as a request to kill the target" standard fare evasive answers like "Well I didn't consider every possible interpretation of my orders; I delegate precisely to avoid such work. I had faith that my subordinate would find a solution that wouldn't cause any legal trouble" could be used to deflect ad nauseum.

3. Characters Immune to Zone of Truth. There are features that defeat Zone of Truth. Off of the top of my head, the 17th Level Mastermind Feature from Xanathar's Guide to Everything prevents you from being magically compelled to tell the truth, and also makes it impossible to tell with magic that you're lying (it appears you're telling the truth instead). Such characters can act with impunity in a Zone of Truth. Even if such characters aren't common, in fact even if only 1 or 2 exist in your setting or its backstory, their existence may dissuade Zone of Truth from being used as the Gold Standard for evidence in trials and encourage more traditional methods. If any heavyweight politicians like, they can make a big stink about a time Zone of Truth failed and effectively lobby for its use to be completely halted in criminal trials "pending further review" which could of course be indefinite. This could happen in the backstory.

4. Combining "1", "2" and "3". If there's an active character in the city who is immune to Zone of Truth, it would make perfect sense for them to be a leader of a shady organization who could be hired to do the dirty work for various groups. Leaders could say "Find someone who can solve this for us" to an underling, that underling infers they need to get the Mastermind, once the Mastermind is told what problem needs solving, he can solve it any way he likes (including passing the dirty work on to his own underlings) but lie about the methods and the trail you can follow using Zone of Truth stops there.

Answer 2

Such spellcasting is costly, rare, and illegal for PCs

Costs of Spellcasting

For the authorities: for high profile court cases it is likely Waterdeep, with access to affiliated churches and arcane spellcasters from the Watchful Order of Magisters and Protectors, might employ exactly tactics as you describe on the accused or witnesses. (Unless they are also bound by the Code Legal in their investigations, see below - I've not found clear evidence for or against so far).

However, this is an expensive undertaking. According to the PHB, p. 159, Spellcasting Services:

Hiring someone to cast a relatively common spell of 1st or 2nd level, such as cure wounds or identify, is easy enough in a city or town, and might cost 10 to 50 gold pieces (plus the cost of any expensive material components).

A simple zone of truth is already level two, falling onto the top end of this range. Dispel magic is level three, and greater restoration is level five, and may require special "service instead of payment" (ibid.)

People who can cast these spells are rare. Higher level spells are much more difficult to come by, as higher level spellcasters tend to be rare. Zone of truth and greater restoration are primarily divine spells. Ed Greenwood estimates that 1 in 6000 of those that work for a church have the ability to wield divine magic and become a cleric that can cast spells. Now, while that ratio will be more concentrated in Waterdeep, that still is a very limited resource.

There are maybe a dozen major temples in Waterdeep. The (third edition) Waterdeep: City of Splendors sourcebook has much more in depth information on the city. It gives exact numbers of clergy for each church, and puts the head cleric averaging around caster level 10, with some of the smaller ones not even having access to level 5 spells. That means essentially not even all high priests are able to cast greater restoration, a very limited pool of people, people who have other concerns than helping the Waterdavian legal system.

It just is not economically feasible to use these spells for most court cases. Yes, powerful player characters may have access to these resources, but if your concern is about coherence and consistency of the game world, always remember that magic -- even for a relatively high-magic setting like the Forgotten Realms -- is rare and exceptional, and does not scale to cover everyday life. This is the same argument as why diseases exist in a world of magical healing.

The Waterdeep Code Legal

For the PCs: casting such spells on citizens of Waterdeep is actually illegal, viz the Code Legal (p. 222, Dragon Heist), Section IV. Crimes Against Citizens:

Using magic to influence a citizen without consent: fine or damages up to 1,000 gp and edict

where Edict means (ibid) "forbidding the convicted from doing something; violation of an edict can result in imprisonment, hard labor, and/or a fine".

Casting zone of truth does influence the citizen: they cannot lie. So unless they first consent, it is illegal. (Detect thoughts however, might be OK, and will serve a similar purpose).

People can decline to comment

As pointed out in the other answers in more depth, the spell has a built-in safety valve too:

An affected creature is aware of the spell and can thus avoid answering questions to which it would normally respond with a lie. Such a creature can be evasive in its answers as long as it remains within the boundaries of the truth.

So, you can just make use of your right to remain silent, or, more insidiously, give evasive or misleading answers that still are not outright lies. While the latter can be overcome with proper cross-examination and careful follow-up questions, the former is not easily solved short of resorting to torture, or other magical means like suggestion.

Overall, you are right that these kinds of spells make whodunnit mysteries harder to run, but the game's authors appear to have had long experience with this, and have provided some way out in nearly all of them. With suggestion you do not know if they made the save, detect thoughts can only pick up surface thoughts and gets a save for probing deeper, detect evil and good now only detects creature types not alignment any more.

Answer 3

Caveat I'm not familiar with the Waterdeep: Dragon Heist adventure personally. Therefore, my answer will focus on how to deal with this in general, as a DM running an arbitrary adventure, and there could be nuances of Waterdeep: Dragon Heist that I'm missing.

That said, let me get on with it.

If I was designing/running an adventure like this, and I didn't want to restrict use of divination spells in general, but I also wanted to allow people to have secrets, especially in a legal setting, I would use the trope of a constitution which restricts use of magic by the judiciary.

The details are variable, so one or both of the following could be used either separately or in conjunction with each other:

• The police and courts are forbidden from using magic to force people to incriminate themselves or violate their private thoughts.
• Citizens have "miranda rights" which allows them to remain silent and refuse to be subject to spells such as Zone of Truth.

Just as in the real world, these rights didn't appear out of nowhere, but rather evolved out of centuries or millennia of seeing what can go wrong when you violate these rules. Also just as in the real world, the rights may not be unquestioningly taken for granted, but you may constantly see authoritarian factions pushing to relax the rules (eg "if they've got nothing to hide why not let us use magic" etc.)

Using the real world as a model, people use their right to silence all the time. The police may not like it, but once it gets to the court room, the police better have their ducks in a row with real evidence collected according to accepted protocols. Otherwise the case gets thrown out.

In a non-judicial setting such as business meetings

If magic is common enough that people can routinely use it to spy on each other, then countermeasures would proliferate. Just as often as someone is using Detect Thoughts or Zone of Truth, someone else is using Detect Magic or Countermagic.

Items such as Ring of Non-detection would also be popular. Fakes might be sold to the poor, while everyone who is rich and powerful would have access to the real thing.

Spellcasters may build businesses around the promise of magical security, similar to cybersecurity in the real world. You might have a sorcerer who is a "magical pen-tester" (penetration tester) who focuses on "red-teaming" the wards of rich clients. Clerics would find new sources of income for the churches, selling divine protection to favored establishments.

People who don't have access to the levels of protection afforded to the rich and powerful would change their habits. Commoners and low-powered adventurers and criminals would adopt practices of opsec (operational security), splitting and classifying information, only giving it out on a need-to-know basis, preserving the juicy secrets for those who have proven they can withstand magical espionage. A thug hired to go break some legs might be compelled to admit to the crime, but they won't be able to say who hired them or why. People might also claim to have been subject to magical compulsion, and how could it be proven that they weren't?

Answer 4

Using divination magic on others is incredibly impolite

Just as slapping a lie detector on everyone you meet or installing spy cameras in the rooms of your business associates is frowned upon in the real world, it would be considered extremely rude to do the same with magic.

If they notice something like this, most people will get offended or even downright aggressive (whether they have anything to hide or not). Higher rank NPCs will simply remain silent and throw the PCs out without comment (and good luck getting anyone of their friends to talk to you now), while lower rank grunts might curse at or even attack the PCs ("What did you just do? You messed with my mind!").

Even more so, in Waterdeep this kind of spellcasting is even illegal, as noted in the answer by Groody the Hobgoblin, adding legal repercussions to the social ones.

This approach still allows Divination spells to be very powerful if used in the right circumstances - stealthily to escape notice, on a captive NPC, or on someone desperate to prove their innocence - while making them a risky or even detrimental option in others, particularly with regards to more powerful or influential NPCs.

I have personally used this approach as a DM in several intrigue-heavy adventures. While Zone of Truth in particular was not chosen by any of my players, it had the desired effect regarding the use of similar spells like Detect Thoughts and Suggestion: They had to specifically set up particular circumstances (using stealth or non-magical manipulation) where such spells could be used without repercussions, while removing them as the default options for every social encounter. The spells remained a useful tool, without trivializing the mystery.

I also made sure to inform my players well in advance how I would handle these kind of information-gathering spells. That way, they were not caught off-guard by the NPCs' reactions.

Another thing I did in support of this approach is to ensure that at least some NPCs had ways of detecting magical information gathering or manipulation, making it even more risky or challenging to use. While not as easy to implement for a published campaign, giving some NPCs access to Detect Magic, Nondetection, or similar magic further limited the circumstance in which divination magic could be applied by the PCs.

This answer focuses on the social interaction aspect. For the criminal trial setting, I refer to the other excellent answers.

Answer 5

The Example doesn't work that way!

Bob's on the trial, accused of killing Alice.

The trusted 20-lvl judge casts Dispell Magic / Greater Restoration on Bob to ensure there are no Modify Memory shenanigans --> then he casts Zone of Truth --> then everyone in the court room can use Detect Magic to ensure that Bob is indeed affected by the ZoT spell's effect.

So far so good, till this moment your shenanigans work. It all breaks down the very next sentence:

Bob then asked "please repeat the exact phrase: I did not kill Alice"

If Bob can't easily say that exact phrase - boom, he's guilty!

No. The correct answer is: "I repeat. You said I did not kill Alice." That is true and a direct answer. No lie told and he did repeat your sentence 100%.

The proper way is asking like a proper lawyer: asking what transpired and then asking something akin to "Did you know that your actions would result in the death of Alice?" followed by "Did you intend to cause the death of Alice?" etc, asking about every single needed prong to prove murder.

The Waterdeep court may not do any of this in the first place

Waterdeep legal code bans using any mind manipulation spells without consent, which then is punished by a legal edict banning the user from ever doing so again.

Section I. Crimes Against Lords, Officials, and Nobles: Using magic to influence a Lord without consent: imprisonment up to a year, and fine or damages up to 1,000 gp

Using magic to influence an official without consent: fine or damages up to 1,000 gp and edict

Section II. Crimes Against The City: Using magic to influence an official without consent: fine or damages up to 1,000 gp and edict

Section IV. Crimes Against Citizens: Using magic to influence a citizen without consent: fine or damages up to 1,000 gp and edict¹

As a result, the judge may not even start casting without getting consent, which at any moment would be withdrawable - and thus useless. So under the laws of Waterdeep Zone of Truth is not allowable in court.

In fact, it might be understood as an act of against the city to try the truth spell in court, as you try to manipulate the process. Possible charges could be [attempted] Forgery, hampering justice, or even treason.

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1 - Waterdeep Dragon Heist p. 222

Answer 6

All spells work, most of them are illegal to use

I'm also currently running Waterdeep Dragon Heist as the DM. I do not find it necessary to restrict Zone of Truth, or any spell in my campaign. I have two reasons for this.

1. The code legal keeps most spells (including zone of truth) from being abused. Your players cast zone of truth, the NPC refuses to speak and goes to get the city watch and reports your illegal activity (as this is a magic to influence violation). If the players want to use this strategy, they also have to figure out a way not to get caught. Also most spells would either fall under the "Assault/Murder" or "magic to influence" categories.
2. The magic system in the D&D world is exception based, the NPC's know that for every spell, there is probably another spell (or magic item) that overrules it. Knowing this, the legal system of the city does not rely on magical means. And if they did, the spell "Commune" would probably be more problematic than "Zone of Truth" anyway.

Answer 7

On making NPCs repeat phrases

Zone of truth states "a creature can’t speak a deliberate lie" and that "Such a creature can be evasive in its answers as long as it remains within the boundaries of the truth."

Asking someone to say a specific sentence can always be countered with 'Sure I can say: sentence'. It's not a deliberate lie because the person is convinced they have the capacity to say the sentence. It might be nitpicky, but I would argue so is making people repeat a sentence. You can't completely avoid having your players know they are being deceived, but you can always avoid giving them information one way or the other.

Using Zone of Truth is inherently distrustful and manipulative, plus everybody has something to hide, so make your NPCs unwilling to work with the PCs if they cast it. If they are enemies make them refuse to speak at all: just because they can't lie, doesn't mean they are motivated to speak. It's easy to imagine a bad guy being convinced that some other bad guy will do much worse things to them than whatever punishment exists for the thing they are being cleared for.

People can't say something about a thing they don't remember or don't know. They can say something untrue that they have been made to believe in some way or form. Either through some kind of illusory shenanigans or the modify memory spell.

Or: turn it around on the players. Zone of Truth doesn't discriminate, make the players incriminate themselves in some way. Make them share compromising information or make them believe they are responsible for something and make them confess to that thing. Or make them look bad by asking them if they have violated a citizen's rights by forcing them to incriminate themself.

Working around Zone of Truth to the point it never gives the PCs any information you don't want them to have is possible. Limiting usability and knowing how and when to respond goes a long way towards keeping the players in the dark.

But that guy who wanted to go crazy with the Zone of Truth spell left the campaign, so I still regret that decision, like I killed his player agency with that ruling

I don't think you could have made another decision at that point, so I feel there's no need to regret that. But it can still serve to make you a better DM :)

Answer 8

It's poor form to negate good choices

I will offer a direct answer to your question at the end, but my post is centrally concerned with respecting agency.

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As a general rule: when somebody uses sound thinking to prepare for challenges they anticipate running into, and then it turns out they surmised correctly and their choices were very good, it is poor sportsmanship to rewrite the rules or disallow those choices to preserve some challenge you were planning to present. This is true whether the "somebody" is a player or a character, but I think it's especially bad if it was a character.

I think your player was right: you did sacrifice their agency, and I'm not surprised they chose to leave.

What is agency? It's the power to make choices that meaningfully affect your fate. If a person is never presented with choices, or they are given choices but they're all meaningless because every option leads to the same outcome, they don't have agency.

In your case, a character made a wise choice that legitimately should have had a significant impact on how events played out, but you decided you didn't want that to happen, so you changed the rules to make their choice meaningless. You retroactively made it be the case that all choices led to a specific outcome that you wanted. They tried to exercise their agency and then you deleted it after you saw that they used it well.

If you were playing tennis with somebody, and when you managed to land a shot they paused the game and redrew the lines so your shot was out-of-bounds, and all the spectators said "yeah, that's fine, your shot is bad now," would you keep playing?

If not now, when?

Think back to the first time you read through the powers of each character class. When you read about an unusual power, you probably paused to imagine what kinds of scenarios it might be useful in.

What kind of scenario did you imagine when you first saw Zone of Truth?

The first thing I thought of was a criminal trial. That doesn't mean it's the only or even best case, but it's obvious that magically preventing lies would be extremely useful in a trial. It wasn't some stroke of brilliance to recognize the utility of ZoT -- it's obviously a good fit.

Disallowing ZoT in this kind of case would be like not letting a mage cast fireball when more than 2 enemies are standing next to each other. It is perverse to neuter an ability precisely because the scenario is an excellent fit.

If you forbid the party from using Zone of Truth when they're talking to somebody they want the truth from, then when do you propose to allow it? Only in cases where it would be pointless?

A character heading to a dark dungeon will bring a torch. A character heading to Waterdeep, a town filled with lies, will bring Zone of Truth. Would you disallow torches?

How to accommodate Zone of Truth

I have two proposals that both draw on the real world, addressing different contexts.

• Trials and other formal proceedings

  Bad guys who know they'll be placed under Zone of Truth at trial will do everything they can to avoid getting placed in that hot-seat to begin with.

  Violent and low-level criminals will be more likely to use the phrase "You'll never take me alive!" The party will find that many baddies, even those engaged in mild crimes like pickpocketing, are surprisingly ready to resort to deadly violence to avoid capture, and will be impossible to talk down.

  Baddies in positions of power (e.g. corrupt mayor or president) will use everything at their disposal to prevent or forestall a formal trial. Rather than get hauled into court, they will work the refs, call in favors, apply inappropriate social pressure, cultivate corrupt contacts in law enforcement, tamper with evidence, have honest investigators bribed, fired, replaced, or even killed, etc.

  If they can't derail the proceedings, they'll delay the trial or at least their own testimony, indefinitely if possible. Or, they might force the prosecution to ask their questions through the defendant's lawyer, who will reword every question so it can be answered without revealing anything. Or they'll insist that the prosecution submit all their questions ahead of time in writing -- and then have their own lawyers write answers and send it back with a statement that the written answers negate the need for testimony at trial. They will do everything possible to replace the magical guarantee of truth with meaningless non-magical guarantees, and they will pressure everybody involved to say that's just as good.

• Social calls and other informal occasions

  Most people will bristle if you begin your conversation with them by using magic to prevent them from lying. Even a person who has no reason to lie will resent the obvious implication that you think they might. By casting Zone of Truth at the beginning of the conversation, the party is explicitly stating that this is not a conversation but an information extraction operation. It is extremely insulting, and I would expect any NPC who notices it to immediately terminate the interview in a flash of righteous indignance. The party will quickly gain a reputation for being extremely rude and suspicious, and folks will naturally speculate this is because the party are themselves frequent liars.

  Similar thinking applies to the trick of ordering the NPC to make a carefully crafted statement. Nobody will stand for it, and I would expect every NPC to immediately terminate the conversation and demand an apology the very first time the player tries this. Even without ZoT, it's very odd and pretty rude to instruct somebody to make a bunch of specific statements. Nobody will tolerate being treated like a trick pony, taking orders, having a rude stranger put words in their mouth, and be treated like some kind of suspect.

Your player thought they could ruthlessly exploit ZoT to cut through all the deceit in the town, but they utterly failed to recognize that the behavior they planned to couple with that spell would never be tolerated by anybody for purely social reasons. Ironically, your player's plan crucially depended on ignoring the fact that NPCs are also supposed to have agency (or at least act like they do).

Your player was right to recognize the utility of Zone of Truth in this context, but catastrophically stupid to think they could run roughshod over all social niceties by essentially clamping a lie detector to everybody's head in every conversation. If you had permitted the spell to work as advertised, this player would had a rude awakening in store.

Answer 9

Others have noted Waterdeep's legal code, and the general impoliteness of using Zone of Truth on the streets, but let's think about your hypothetical court scene a bit. I can think of a few reasons why this would be unlikely to play out so simply within the worldbuilding.

Not everyone can detect magic

While detect magic is relatively common as a level 1 spell, the majority of people in the world can't cast it. Unless everyone with any power in the world is a spellcaster, this is going to cause problems. The fighters, monks, (honest) rogues, commoners, tradesmen, etc. in the world are not going to be happy with a legal system that requires them to have blind faith in an outside group.

If I was a nefarious sort of person, I'd organize non-magic users. Sow distrust of magic users ("they say it's 100% effective, but how do we know?), and use any graft or corruption as a cudgel.

It doesn't work well with non-compliance

Our mafiosos can help this process along by getting everyone in their sphere of influence to just not comply with the process. This is something that the process has no real way to deal with.

Sure, if a person or two doesn't play along, you can just falsely imprison them and say that it's their own fault. But if you're in a position where your prisons are full of every dragonborn who's crossed the path of a magistrate you start to lose standing.

This only works for simply cases anyway

This only works for one type of case: The facts are absolutely agreed to by all parties, and one person is blatantly lying.

But there are many situations where this is not the case. For example, if Alice and Bob were fighting, and Alice is dead, but Bob claims self defense, what's your special phrase?

"I did not murder her?"

Well, Bob might be able to say that, because in his world-view it's not murder if the subject had it coming. Or it's not murder if the subject was a member of the other side of a conflict. Etc.

"I killed her in self defense?"

Better, but that only proves what Bob believes at the time of trial. Which may not be true, and may not reflect what he felt at the time (human memory being, by its own nature malleable).

What phrase would you use for the classic Mafia dodge of turning to an associate and saying "I need you to take care of our Alice problem?" The boss isn't killing Alice. He's not asking to have her killed (within enough of a technicality for the spell). How would you entrap him with Zone of Truth?

And what about the other direction? History is full of people who have confessed because they were convinced that they were responsible (either directly or indirectly). Getting someone to believe they're responsible for a death, and therefore "killed" the subject is not an impossible task for an aggressive prosecutor. Even if that person doesn't believe that they wielded the knife themselves, the english language is flexible enough that they can still satisfy the sentence "I killed Alice" (by driving her out of the safety of the inn, and to the dark alley where she passed away).

You assume the people who made the system have nothing to hide

The biggest barrier to this from a world-building perspective is simply that the people who set systems like this in place tend to have secrets of their own.

Richard III would certainly never have allowed a system to exist where he might be asked about the fate of hit nephews. And he's probably not going to let that system ask other people questions, because he doesn't want to create an expectation that he'll be asked.

Other nobles will have similar reasons for hiding things. And therefore similar reasons for ensuring that this system doesn't exist (or that they are insulated from it).

It still assumes you can catch the culprit in the first place

Regarding "can intrigue work in a world with Zone of Truth," this still doesn't change one fundamental truth: if you want to do dirty deeds, it's better not to get to trial in the first place. Whether by concealing yourself through intermediaries, or just bribing and corrupting your way out of imprisonment.

Justice systems don't compel powerful people lightly. If a shady person is being brought to court in the first place, they are probably already losing.

Answer 10

While a character can refuse to answer questions while being affected by Zone of Truth, doing so can put suspicion on them. An innocent character can exonerate themselves by stating their innocence, so a guilty character would immediately appear guilty if they refused the opportunity to do so.

As a counter-measure, I once created an NPC that made himself practically immune against "Zone of Truth" and other means of compelling him to speak the truth, by having him swear an oath:

I swear to [chaotic god associated with treachery] to not answer any questions while I am magically compelled to tell the truth.

Now while under a "Zone of Truth" spell, he would reply to every question with:

I am sorry, but I swore an oath to not answer any questions while I am magically compelled to say the truth.

Which is completely true, but unhelpful. In order to find out more about why he swore this oath, he would need to be released from the ZoT. Then he would present a story about it being part of his religion and that he would totally want to state his innocence while under ZoT if possible, but he would be more afraid of being punished by his god than of being punished by the law. He was free to lie that it was just a ploy to give him a truthful reason to evade questions.

Answer 11

Detect Thoughts, Commune/Divination/Scrying, Zone of Truth, and investigation heavy adventures.

These spells, and similar effects, make investigation heavy games more difficult or impossible. Jumping through hoops to design scenarios that cannot instantly be solved by abilities like these can be hard, especially for storytellers more used to hack and slash adventures.

Simple Fix: Unreliable Information

If spells exist that stymie other spells that read minds, either by fabricating memories, confusing recollections, or hypnotizing the target and these effects are accessible, perhaps in potion form, via rituals, or using natural magical effects (magical flowers that send someone into a suggestible state are a staple of fantasy), and these methods are often used by intriguers, criminals, and even wary adventurers to avoid having information ripped from their living brains and used against them, then the information received by these spells cannot be directly trusted. To make this clear, have information be gained by Detect Thoughts and then be false in a way that costs the party something minor - money, reputation, time - and later the party's efforts uncover the method by which their spell was foiled - a small jar of powder, a book detailing a ritual, which makes it clear this is not some grand powerful ability in their way but a common one used by many.

Likewise if spells, effects, or actions can simply stymie divination (such as rolling a die to determine when you do something, having some associates use the same name as you to confuse divinations that identify you by that name, using a false name so divinations fail if directed at you, etc) and the information that is acquired using that method is unreliable, then the party will hesitate to use even correct information due to (entirely plausible) fear that it is more incorrect information.

Simply denying the information ('you get nothing from your spell') when defensive measures are used, and handing out perfectly accurate 100% reliable information when defensive measures are not used or the party get around them is less effective than making information often wrong. That will introduce far more hesitation and actual investigative measures.

I talk here about defensive measures people can take, but you can also simply make the spells inherently unreliable and prone to giving false positives. The mind is a complex thing, as is fate and destiny. That a fourth level spell or a second level spell can 100% reliably unpick these incredibly complex things is not a given and giving them a (secret) chance to give bad info instead is perfectly reasonable.

Simple Fix: Defenses Exist and People Use Them

Someone reading your mind is not limited to solely gaining information that they use to save the city from an evil plot. They can also see your infidelity, where you've hidden your gold, your fears (to later use against you), your hopes (to use against you), your weaknesses (to later use against you), and your strengths (to later use against you). Likewise if they play Twenty Questions: You Edition with Divination. There is no reason someone is using this power morally and responsibly.

Ergo it's illegal. On top of that, many people carry cheap and easy to procure magical items (that require attunement) in any medium to high magic society (like Waterdeep) produced en masse by the local temples so that people can't just rummage through their minds at will. You need a court order to rummage through someone's mind or use divination to uncover details about them, their actions, intentions, etc. It takes time to get the court order - without pre-existing proof the judges are unlikely to grant it. Even if they do, you might need to arrest the person to force them to take off a magic item protecting them, and if this happens for someone who is innocent you might get in trouble, have bad PR, or even get locked up yourself.

Simple Fix: These Spells Don't Exist

You can play a fantasy game without mind reading and Dimensional Google. It's fine. You are still shooting flames, riding winged horses, and teleporting through space. Much fantasy, very wow.

Mix and Match

Nothing stops you removing Commune & Divination, making Augury and Detect Thoughts give unreliable information, and have Zone of Truth made illegal because of the horrible side effects. None of these options are mutually exclusive, except removing all spells that do this.

Answer 12

As far as I know, you can succeed the saving throw and only the caster of the Zone does know you succeeded. But this loophole was fixed by your detect magic solution. My ideas would be.

Limit it

Make it high level (7th level), with a week casting time and with costly components like 5000 Gold pieces.

It could be used in very high stake cases, but won't be used for anything lower than the killing of a noble.

Limit the zone to questions and limit the number of questions which can be asked in the zone to three questions. Limit the words and letters a question can have (like six Words with 50 letters max).

Rule that the zone works only on questions not commands like "speak the following sentence: I have no idea or involvement in the murder of the duke of Oakmountain". The zone would be used in this case: Did you murder duke Oakmountain? (32 Letters, 5 words). But after a no, you only have two more limited questions. So if Baron Darkfield paid a servant to hire a unknown murderer, and would be asked "Do you know Oakmountain murderer?" and "Did you pay Oakmountains murderer?" he could say "No" both times. This won't make the spell useless but way weaker. You can increase the number of words and letters, to make it easiert for your group. A good question in the above example would be: "Did you want Oakmountains dead?"

Consequences on your world

If the case is complicated, you can't get to truth with only three limited questions. This means you have to wait an other week and spend 5000 Gold coins, hoping three more questions would reveal something useful. And if you got the wrong guy, you wasted 10K gold and 2 weeks of an archmage for nothing. Which might cost you your job, your reputation or even your head. Also if there are 5 suspects you can't put them all into a circle and call it a day. If you have to wait a week, the real murderer could already have left, while you are waiting for the mage to finish the ritual.

The dark eye has a similar single target spell, which states "the victim of the spell must answer truly, but must only answer with yes, no or silence if the answer is unknown". This way the things you can extract are way more tedious, like a game of 20 questions, where each question costs time and money.

Or just ban the spell.

Answer 13

There are some options for you besides what others have already stated:

• A warlock can have an invocation that allows lying even inside zone of truth

• A REALLY good liar can convince themself that what they say must be true. This is a form of cognitive dissonance but you can train it. "The best lie is the one that you believe yourself in the end" - my grandfather

• You can modify memories, basically what Slughorn in Harry Potter and the Half-Blood Prince did. That way it's technically not a lie anymore.

• Powerful curses can kill people / deny their ability to talk. I would have several organisations with a code of conduct that forbids you to talk about certain secrets. Since everything said in a zone of truth must be true, people are in a dilemma and would rather state, "I'm legally bound not to talk about said incident."