I have some players that I know are going to try and force some information out of a hostile NPC that they just recently killed and use the information to target another group and call for their arrest and/or execution. I'd like to make it as difficult as possible, or even just flat out have the head (they removed the head from the body) just lie or give evasive answers.
But one of them has Zone of Truth and I know that they'll attempt to use it on the head once Speak with Dead is cast. Can they force honesty with Speak with Dead/Zone of Truth in this way? Does the head still count as a creature?
Best Answer
No, zone of truth does not work on corpses.
Speak with dead targets a corpse:
A corpse is not a creature, it is what is left after the creature dies. And according to an unofficial Twitter ruling by Crawford, the corpse is actually an object.
Zone of truth only targets creatures:
So a corpse, even one under the effects of speak with dead would not be affected by this.
Another unofficial twitter ruling by Crawford supports this for this exact combination of spells:
Even if you did, for some reason, consider a corpse to be a creature, it still would not force you to reveal information honestly. Firstly, it only has an effect if the creature fails a charisma save (you'd have to decide what this was since the corpse is not a creature anymore and doesn't have stats). Secondly, the corpse doesn't even have to try to answer the question helpfully. There are plenty of ways to be vague or cryptic while still being technically truthful and thus obfuscate the information given to the PCs.
Of course, in-universe, there is not necessarily a reason that the corpse would automatically consider the PCs enemies and avoid giving them truthful answers. So, if you want them to avoid answering be sure they have a reason in-game to do so.