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?
Best Answer
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:
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.