D & D 5th has a handful of spells and features designed to protect the user from the magic of lying detection. The examples I am aware of are the glibness spell and subclass of Soul of Deceit of Brain Thief. More precisely, glibness said:
[…] No matter what you say, the magic that determines if you tell the truth indicates that you are truthful.
Soul of Deceit provides the same functionality, but goes one step further:
[…] whatever you say, the magic that would determine if you tell the truth indicates that you are truthful if you wish, and you can not be forced to tell the truth by magic.
At first glance, these "anti-lie detector" features seem designed to defeat Truth zone. I certainly thought so when I wrote this answer. However, as rightly pointed out the comments on this answer, Truth zone is not precisely a lie detection spell, and the precise interaction between this spell and these features is unclear. I will read the entire spell carefully because all of this could be important:
You create a magic zone that protects against deception in a sphere of 15 feet radius centered on a point of your choice within range. Until the spell ends, a creature that enters the spell's area for the first time on a turn or starts his turn must make a save charisma roll. On a failed save, a creature can not deliberately lie in the ray. You know if each creature succeeds or fails their save throw.
An affected creature is aware of the spell and can avoid answering questions that it would normally answer with a lie. Such a creature can be evasive in its responses as long as it remains within the bounds of truth.
In short, a failed backup has the following consequences:
- Affected creature can not lie
- The affected creature is aware of the compulsion
- The caster knows that the creature failed during its backup.
while a successful backup only alerts the caster that the backup has been passed. It is important to note that each creature in the spell's area makes this save roll even if it is immune to all effects of the spell.
The problem is that Truth zone does not tell the caster directly whether the affected creature is lying or not. It tells the caster if the creature failed its save throw against the spell. So, despite the spell's name, it's unclear whether it's "a magic that would determine if you're telling the truth". The additional clause of Soul of Deceit clearly seems to have to overcome the compulsion of the spell against the lie, but beyond that, it is difficult to know what other effects, either Soul of Deceit or glibness protect against.
So, assuming the creature fails its save roll and is affected, how far glibness or Soul of Deceit protects the affected creature from the effects of Truth zone? Specifically, does either of these means allow the affected creature to tell a deliberate lie, and if so, what information does the caster Truth zone receive when they do it, and how do they differ from the information that the caster would receive if the affected creature did not have the effect of protecting them?
Note that I do not ask about the affected creature's ability to be elusive or avoid answering a question as described in the last paragraph of Truth zone. Being evasive is always allowed by fate. Suppose the affected creature wants / must tell a deliberate lie.