I had a very interesting discussion with my players on the effects of the Zone of Truth in D&D 5e.
They have committed a murder / killing in self defense and will be tried for it and we talked about whether the Zone of Truth is balanced.
We were discussing the use of Zone of Truth as part of a court process.
As a DM, I think this spell is either always OP or always useless according to its interpretation:
All cases take into account that the caster knows if the target of the spell resisted the spell, therefore it will not be used in court unless the affected person didn't resist. Furthermore, if such resistance could be willingly forgone, any court would condemn immediately any resisting person. So we are talking about when the spell is not resisted.
- In the case that it only prevents one from speaking false statements (as far as they know) but doesn't compel one to answer the question presented to them, they can always detach themselves from it and answer a different question that they ask themselves within their mind (the same technique used against a lie detector machine). This will make the spell useless.
- In the other case in which one is compelled to answer the question that's presented to them in its exact formulation, there is no way of not telling the truth as long as the interrogator follows a simple algorithm:
- Ask a question as specific as possible. (The affected can withhold parts of the truth.)
- Ask "Did you hide any information that you think is relevant while answering the last question?" (The question is a subjective yes or no question and refers to the information the affected tried to hide, therefore they have to not answer, or confess hiding information. The affected can only hide information if they subjectively think it's relevant).
- Ask "Will you tell us now the information you kept from us before without withholding any relevant information?" (Another yes or no question that by answering either condemns or compels the affected to keep to the truth, because if they know they're going to withhold information they are unable to say yes because that would require them to believe they will tell the truth. During a timeframe so short it's not possible change their belief about telling the truth without a reason.)
- Repeat step 2 until the answer is no and no more information that the affected knows is relevant is left unsaid.
- End with "Do you know any more relevant information that was not discussed here that would prove your guilt?" (ending the interrogation only if the answer is no) This question is to reveal the existence of information the affected thinks is relevant but was not asked about.
The philosophical discussion is very interesting:
- Is it possible under the effect of Zone of Truth to answer half of a question?
Q: "Do you like red and blue?"
A: "I like red."
Here the question was referring to both red and blue and answering only to one part of it would also allow for the following:
Q: "Did you kill the king?"
A: "I did not" (kill a chicken)
Or must the answer regard the full question?
Q: "Do you like red and blue?"
A: "I do like red but don't like blue."
My players were saying that Zone of Truth is balanced because it's the intelligence of the affected that prevents them from splitting a question "Did you kill the king?" into "Did you kill?" or even "Did you?" while not binding the affected to answer the question presented, because the affected knows the question was about the king and can't deliberately answer "no" when they know they did.
To that my answer was that the same intelligence would prevent the affected from using technically truthful statements when they know they are falsifying the truth.
So it all comes down to the basic question:
Does Zone of Truth only deal with technically true/false statements, or does it screen the mind of the affected for any subjective truth/lies?
Best Answer
Zone of Truth is still useful even if it works according to your first interpretation
To be honest, if the king's murderer is asked the direct question "did you kill the king?", and they respond "no", then most people are going to rule that as a lie, even if murderer is cunningly ignoring the question they were asked and "actually" saying something completely unrelated. However - even if you allow this kind of shenanigan to get around the zone of truth and don't consider it a lie - a skilled interrogator can still effectively use the zone of truth to compel truthful statements from their subject, and thus arrive at the truth (if only by implication or omission). The trick is not to allow the subject to respond with ambiguous or unqualified sentences, but require them to make a full statement which does not depend on context.
Say our murderer, Alice, has killed King Bob, and knows full well she did it. Under the effect of a zone of truth, Alice cannot say "I did not kill King Bob", because she did, and that would be a lie. Our interrogator, Eve, knows exactly how the spell works, and so she decides that the only statement Alice can make which will prove she did not kill King Bob is "I did not kill King Bob." Rather than ask "Did you kill King Bob?", knowing that Alice is tricky and can weasel her way out of a direct answer to the question, Eve instead gives Alice a simple instruction - "Say the words 'I did not kill King Bob'."
Now Alice is trapped. She cannot say such a thing, because it would be a lie. But of course, someone who did not kill King Bob would be able to say that immediately, and so prove that they didn't do it. But Alice cannot - and so she cannot satisfy Eve's demand. She might say a lot of other things, attempting to distract or confuse Eve, but so long as Eve sticks to her guns and requires that Alice simply and definitively say that she did not kill King Bob, Alice's inability to say so is strong evidence that she did kill Bob.
The zone of truth still has a hole in it, though. A subject under the spell cannot lie out loud, but they can lie by omission, in the sense that they could refuse to say things that are true with the intent to mislead their interrogator into believing those things are false. For instance, if Alice did not in fact kill Bob - but she knows who did, and is willing to go down to protect them - she could implicate herself by refusing to say she didn't do it, and thus divert suspicion from the real killer.
Ultimately, the zone of truth will guarantee that any independent statement the subject makes is one they believe to be true. If a subject cannot - or will not - say a certain thing, it is strong, but not incontrovertible, evidence that they don't believe that thing to be true. A skilled investigator who keeps these things in mind will still find the zone of truth to be a very useful tool in their arsenal.