Creating a city made of illusion that won't be quickly figured out

Question

We are playing DnD5e in custom world. This is going to be my first time as DM. Currently I'm trying to get my first adventure together. My idea was to trap my players in the city, which is just an illusion made by Very Evil Necromancer. Just on the very end the players are going to dispel the illusion, and see what they have done.

There is however a problem. First, saying "ok, throw for perception" just when entering the city is going to raise suspiciousness among players, which I do not want. Not even speaking that one can throw 20, and see through illusion at the very start. Also, Detect Magic is going to show them a presence of it.

In PHB, Detect Magic is low-level spell, which is sure to be picked up by at least one player. It shows not only if something is magical or not, but also what school of magic it has. So, "I cast detect magic on this wall. What do I know? - It is magical, magic is of illusion school - There is something hidden there! Lets dig it out".

I do not have my own Dungeon Masters Guide, as I had to lend it to someone, and I can not check, if there is any rule, specified in any official material, that could hide the presence of illusion or any similiar magic. Of course, I could just handwave it away "You were way too weak to see through this" but I want to keep away from such techniques, as I find them unfair.

Is there anything in rules, including drafts from Wizards, that would allow me to create this city without fear that players will find out prematurely? Some kind of detection jammer? The players are quite experienced, and oldest of them was already playing when I was trying to figure out the connection between sounds and letters. Characters are low-level however, not higher than 4 levels. There is bard and warlock, and one more character of unknown class by now.

What I exactly want to achieve: the city was destroyed by an army of undead creatures. The buildings are partially destroyed, but more-or-less in the same shape, so walls are where they were - no need to care about character falling through walls. All green pastures are now ash. Five holy temples of some non-good deities are however immune to magic of evil necromancer, and Necromancer wants them out of the way. To achieve this, he covers the city in illusion, so coming-by heroes are convinced, they were given a quest to save virgins from evil cultists. This includes undead shopkeepers, bartenders, and so on. Necromancer himself is way too powerful to deal with, and this is beginning of long campaign, which will ultimately lead to alliance with said necromancer against The Greater Evil From Beyond.

Answer 1

First of all: Welcome to DMing! It can be super fun and rewarding, and I hope you enjoy it.

One of the things that most DMs learn when they're playing a game that works like 5e is that you don't always have to follow the rules. If it would improve your game, you can totally rewrite or ignore parts of the rules. This is usually referred to as Rule 0, and it can help you out a lot here.

It sound like you have 3 goals:

• Initially, trick your players into thinking that the city is functional.
• Allow your players to figure out, over time, that the city isn't actually functional.
• Set up your Big Bad Evil Person as a powerful spellcaster.

It's totally allowable to say that the Big Bad Evil Person has access to some kind of powerful illusory ritual that lets him cover the city, and helps you with these goals. The way that I'd run it is this:

The Big Bad Evil Person knows the ancient ritual, Illusory City. This ritual is more powerful than most spells, and takes lots of time and special materials to perform. It covers a mostly-intact city with an illusion that makes it seem like it's whole, as long as the buildings are mostly standing. Any designated creatures in the city when the spell is cast are also covered in this illusion, and will look like average citizens of their size. For example, a zombie could look like a bartender, but wouldn't gain the ability to speak or tend bar, but a vampire would no longer look deathly. A Detect Magic spell cast within the city will detect a vague aura of illusion magic, but won't reveal that everything in the city is covered in an illusion. This applies to any Detect effect in the area; a Paladin using Divine Sense would detect that there's undead in the area, but wouldn't be able to pinpoint the number of sources, or their location.

This is still just an illusion, so anyone touching an object, creature, or building affected can make an Intelligence (Investigation) check (DC 20) to figure out that it has an illusion on it, but this doesn't dispel the illusion, or let the character see through it.

This spell description aids the first goal by making it non-trivial to figure out that the city is an illusion. If they cast Detect Magic, they get valuable information; namely that something super weird is going on. :) It serves the second by allowing several ways to discover the illusion: touching something an a check, or interacting with a non-intelligent undead "citizen". You could also maybe let the characters smell the rotting corpses, without telling them exactly where the smell comes from. This description also serves the third goal, because it's a kind of magic clearly out of the scope of what the players can do.

By using a homebrew spell like this, with clearly defined rules and ways to interact with it, you can make up something that fits your design goals while avoiding handwaving away the player's abilities.

Answer 2

Okay, first off, do not have them roll perception to see through the illusion unless your players are actively trying to see through it. Players have a passive perception score, which was designed specifically for this purpose.

Secondly, there is no way you can hide uses of Detect Magic without hand-waving it once your players question it. However, castings of Detect Magic are, in my personal experience, not very common. My current campaign has had a grand total of two castings of detect magic, and the wizard is level 6. Besides, from the current classes, the bard is the only one that can take that spell, and bards get access to a lot of interesting level 1 spells, and can only choose a limited number of them.

The only way to hide magic within range from the caster is the ways outlined in the spell itself. Covering the illusion in a thin sheet of lead, for example, would cover it perfectly. This doesn't help with illusory repairs of buildings, or undead with illusions to make them appear living.

I would personally suggest not worrying about it until they actually cast Detect magic. Do keep in mind that Detect Magic has a range of 30 feet, though. They have to be relatively close to the illusion to actually see it.

There is, of course, also the option of creating a new high-level spell that hides the magical auras of anything it is cast on from spells like Detect Magic.

Answer 3

Where in the rules does it say that you can use a Wisdom (Perception) check to discern an illusion?

Spells and abilities in D&D 5e do exactly what they say they do; nowhere in the description of Wisdom (Perception) is detecting illusions mentioned at all! It is not in Intelligence (Investigation) either. You are ascribing powers to these proficiencies that they just don't have.

I suspect that you have played earlier versions of D&D where illusions could be "disbelieved". There is nothing like this in 5e!

How you can detect an illusion and what effect this has, if any is spelled out in the description of the particular spell. So, for example, if you used Mirage Arcane to create the city then the only thing that allows detection is truesight or, possibly, tasting the terrain since this is the only sense not covered; however, how you could tell the difference between ruined masonry and unruined masonry by taste is beyond me. Specifically, with this spell, truesight allows you to see both the real and the illusionary but you still have to deal with the illusion!

Similarly, if you used Major Image or Programmed Image as the basis (some modification required but not much - perhaps mixing with Alter Self) for "skinning" the undead then physical interaction is required because "... things can pass through it", however, they couldn't pass through the underlying creature so it wouldn't exactly be obvious.

Both these spells allow a PC to take an action to roll an Intelligence (Investigation) check against the caster's spell save. Clearly, they have to be suspicious before they would do this - noticing that they coins the shopkeeper handles go partly into his skin (DC20? DC 25? Wisdom (Perception)) might trigger this suspicion. If they succeed they can then see the reality underneath for that particular spell - all the others are unaffected. Will the be suspicious enough to check everything? If so, good for them!

As for Detect Magic, you have 2 choices:

1. Rule that one thing all illusions do is hide themselves from this particular spell. This is a reasonable ruling as it is silly that a 1st level spell can nerf illusions of levels 2-9. As a refinement, allow Detect Magic to detect illusions of the spell slot expended or lower.
2. Let it do exactly what it says - the city reeks of illusion magic. In what way exactly does this invalidate the illusion? The PCs know there is an illusion here but what is real and what is illusion? They are really no closer than they were before, except now they are suspicious and can take their action to try and see through the illusion.

Answer 4

To address the problem of (unintentional) metagaming when you ask your players to roll perception. Before play starts, ask what their perception score is. Roll for them, perhaps before the game even starts if it is a predictable event like them visiting the main city.

To protect from detect magic, you could have something obvious there for them to detect - a sacrificial lamb.

PC: "I use detect magic"

DM "You detect 300 illusion spells - roll perception/will save/knowledge local, as appropriate."

PC: rolls 15

DM: Those new statues commemorating the recent victory over the evil army of badland look a bit strange to you. You don't think the city coffers would have been able to stretch to buy that much gold.

PC: "That cheating architect's guild must be stealing money. Begins sidequest.

Chances are the players aren't going to confirm that there are only 299 statues and wonder about why there are 300 spells of the illusion school.

Answer 5

1. Congratulations, that's an awesome premise. Have fun! (And let us know how it turns out).

2. It's expected, indeed beneficial, to make up some magic the occasion. DuckTapeAl's answer has a very good description. A high-level spell which is hard to see through is fine. The key with making something up is that it doesn't break the player's expectations too much -- if there's a spell that just says "and detect magic doesn't work", it can seem like the players shouldn't bother playing the game, should only guess what the GM intended, so ideally the players usual tactics and spells should still function close to the usual way but not give everything away.

3. It's probably more fun if the players rapidly realise SOMETHING is wrong, and then spend a session finding out what it is, than if a reveal is sprung on them later. Allow detect magic to say "the street is dense with illusion magic", and again if they try it in a different street.

4. Be flexible. Ideally, have the adventure work if the players realise its illusion city almost immediately, or are clueless until the end. This is easier than it seems, and the players won't notice, they'll just think you planned roughly what happened.

5. You know roughly what the players perception is in advance (either their passive perception, or their likely roll). Neither "you didn't notice anything" nor "you realise the whole city is an illusion" is fun. Instead, design one (or more) acceptable outcomes and plan for those -- eg. assume the players detect SOMETHING is wrong, but find it hard to see what, or that if they examine an item/NPC/place closely they get a chance to see through the illusion, but only over time do they get the "oh shit" moment that EVERYTHING is an illusion.

6. Related to #4 and #5, a big reveal that the PCs have been fooled can be powerful, but is also best used in small doses. If you haven't already, consider ramping up the clues that something is wrong until it's spelled out for them, but act like the PCs "figured it out".

7. Also consider other actions or spells the PCs can take which might reveal a problem. Touching someone? Detect evil, detect thoughts? Plan what will happen (ideally the PCs get another clue without necessarily learning the whole give-away).

Answer 6

You don't have to build the entire environment using one single spell, the fact that the city is an illusion does not have to be the "big reveal". It can be something more subtle than that, and more awesome. A very high bardic knowledge roll could blow whole thing before they have even left the tavern (the bard will feel very clever if that happens.)

You stated that this city was built around the ruins of a real city - perhaps most of the city is real, but its nature is heavily concealed, twisted, and warped, with various layers of illusion spells which have to be investigated or, failing that, dispelled, hit repeatedly with sharp objects, and so on, to chip away at the layers. Over several sessions the party can solve problems, fix things, save people, kill minor baddies, etc.

The players guide lists several normal illusion spells which can achieve this effect. Minor illusion, silent image, disguise self, hallucinatory terrain, and more. Something can be casting illusions in real time; perhaps it is a minion of the boss, sent to delay or thwart the party. The 5 story tower with a sign on top reading "free healing potions"? Not there. The hole in the road with tentacles at the bottom? Fake, that path is perfectly safe. The two trolls with jester hats, juggling on a street corner? Totally real.

Make each challenge / puzzle interesting to deal with, whether or not they succeed on their investigation rolls. You could get many sessions out of the different quests which could be required to "fix" different bits of the city. Eventually the party will have cut away at the layers and revealed who is responsible, ready for the grand finale.

Regarding detect magic; By my calculation, if using all of his or her spell slots, a level 10 wizard would be able to spend 150 minutes per day walking around, pointing out objects with an aura of magic. However, if lots of things within the city are obscured, the spell must be rationed, even if you have said wizard on your party.

Answer 7

And what about hiding in plain sight? (No, not this one).

In a magic world like D&D, where even the lowest level Sorcerer or Wizard has a handful of Illusions and, as mentioned, Detect Magic at hand, it should come to no surprise that magic is heavily used and relied on.

So, I imagine the following dialog when arriving at the city doors:

• Guard: "Hoy, first time here?"

• Party Face: "Yes, we have come to [...]"

• Guard: "Well, don't go using magic nilly willy now, folks like to hire Illusionists to pretty up their shops and all 'round here and they'll be mighty pissed if you damage them; it ain't cheap you know"

Note: I don't have the D&D 5 books at hand, but in D&D 3.5 the spell casting services were relatively cheap, at 10 gp for a 1st-level spellcaster's spell.