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I am planning for a potential trial-like encounter, but ran into this potential snag regarding the zone of truth spell. The question is best illustrated by an example:

Suppose an inquisitor NPC wants to maximize the probability that the PCs are telling the truth, and has two paladins sequentially cast two zones of truth in the area where the PCs are bound.

PC #1 succeeds on the first saving throw, but fails the second. PC #2 does the opposite, failing the first saving throw but succeeding on the second.

Which PCs are bound by the zones of truth?

PHB p. 206 (or this portion of the basic rules) states,

The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect—such as the highest bonus—from those castings applies while their durations overlap.

Does this mean that only one of the zones of truth can be active at a time, and whichever creature made the saving throw for that particular casting is free to tell lies?

If casting zone of truth multiple times doesn't work, would other, similar methods work instead? For example, would forcing the PCs to drink truth serum (DMG p. 258) and casting zone of truth improve the probability of success?

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    \$\begingroup\$ Merle Highchurch, is that you? \$\endgroup\$ – sirjonsnow Jan 8 '18 at 15:15
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Given that spells do not stack, multiple Zones of Truth have no additional effect aside from redundancy of the effect. Failure to save against any zone means they will be under the effect, eliminating all successful sources of Zone of Truth will free them.

As per the handbook, when casting Zone of Truth:

You know whether each creature succeeds or fails on its saving throw.

If the Zone of Truth is resisted/saved, the Paladin will know, so the redundancy might not be necessary and The Zone of Truth can be dismissed and another applied until one succeeds. Applying multiple Zones might save on recasting time should someone manage to save against most of them, but the effect itself is binary and knowable so that approach isn't really necessary.

Given the full knowledge of the effects of the spell, I would not bother with rolling it at all. Establishing Zone of Truth as active on all targets is a part of the trial proceedings that cannot be evaded given sufficient time to do so.

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The one with the highest DC applies as this is the “most potent effect” - if the DCs are equal it doesn’t matter: flip a coin. If that spell expires first then you make a saving throw for the other one.

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    \$\begingroup\$ Dale, the "flip a coin" is not an official rule, right? I think if the DC did not matter, then the longest one could be considered the most potent - notice that the -such as the highest bonus- part means the one with the best parameters. (with an added bonus of saving an extra saving throw). \$\endgroup\$ – Mindwin Jan 8 '18 at 13:05
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    \$\begingroup\$ @Mindwin There can be edge cases where which spell is important and their effects are otherwise identical (exactly the same remaining duration, in this case, achievable with multiple casters). For example, if you have bonuses against spells cast by devils then it's important if one of the casters is a devil and the other is not. Your bonus belongs to you and not the spell, after all. So you still have to clearly decide between two spells with identical potency. A coin flip, though likely not an official rule, seems the only sensible option. \$\endgroup\$ – zibadawa timmy Jan 8 '18 at 16:50
  • \$\begingroup\$ The actual technical game term would be "roll a D2" I think. \$\endgroup\$ – Doomfrost Jan 11 '18 at 14:09
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You are correct when you say that only one zone of truth can be active at a time, however you could get the behaviour you want by having 3-4 castings of zone of truth slightly spaced out, so their edges are up against each other instead of overlapping, with their central point being on the trial. That way you could move the PCs between the zone, asking them the same question each time.

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So I just read the actual text of Zone of Truth for the first time.

And it includes this:

Until the spell ends, a creature that enters the spell's area for the first time on a turn or starts its turn there must make a Charisma saving throw.
On a failed save, a creature can't speak a deliberate lie while in the radius.
You know whether each creature succeeds or fails on its saving throw.

And in fact, nothing in this spell says they only make one saving throw.
Even better, a success does not allow the creature to lie - it merely changes nothing.

So by RAW, a creature inside the Zone of Truth will make a saving throw every round (because it starts every turn inside), and once it has failed a single save it can no longer deliberately lie for the remaining duration.

And, given that the caster knows whether a creature succeeds or not, your paladins need only wait until everyone has failed inside a single zone - they don't even need to worry about what happens with overlapping zones.

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