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The Archmage NPC casts Mind Blank on itself before entering combat per the MM. Mind Blank is powerful enough to foil a Wish spell to gain information about or affect the mind of a target, per the PH.

How does this affect a Zone of Truth spell? Would the Mind Blanked creature have to make a saving throw? Would the Zone of Truth caster be aware of a failed save? Would the Mind Blanked creature be aware of the Zone of Truth?

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Mind blank has no effect on zone of truth.

Zone of Truth (PHB, p. 289):

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.

Mind Blank (PHB, p. 260):

Until the spell ends, one willing creature you touch is immune to psychic damage, any effect that would sense its emotions or read its thoughts, divination spells, and the charmed condition.

(emphases mine).

Zone of Truth isn't a divination spell (it's an enchantment) and doesn't allow the caster to sense emotions or read thoughts. Instead it magically restricts what an affected creature can say; rather than detecting lies, it just prohibits them from being spoken. The effect that lets the caster of zone of truth know whether a target has saved or not also doesn't meet any of these criteria. So, mind blank confers no benefits of any kind when being subjected to a zone of truth; a mind blanked character still needs to make a saving throw, and is subject to the effects of the spell if they fail.

Would the mind blanked creature be aware of the zone of truth?

An affected creature is aware of the spell and can thus avoid answering questions to which it would normally respond with a lie. Such a creature can be evasive in its answers as long as it remains within the boundaries of the truth.

(Zone of Truth, PHB p. 289)

Any creature affected by zone of truth is aware of it; again, mind blank doesn't change this.

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Zone of Truth works normally, except the caster doesn't know if the creature succeeded on its saving throw or not.

The main effect of Zone of Truth (PHB, p. 289) is the following:

On a failed save, a creature can’t speak a deliberate lie while in the radius.

The spell Mind Blank does the following:

Until the spell ends, one willing creature you touch is immune to psychic damage, any effect that would sense its emotions or read its thoughts, divination spells, and the charmed condition. The spell even foils wish spells and spells or effects of similar power used to affect the target's mind or to gain information about the target.

Mind Blank doesn't protect against an effect preventing speaking lies, so the main effect of Zone of Truth is not affected.

However, Zone of Truth also does the following:

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

This effect is blocked by Mind Blank, as it is used to "gain information about the target".

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  • \$\begingroup\$ Your contention is that a compulsion preventing a creature speaking a lie is not an effect "used to affect the target's mind"? If so I think elaboration on the logic behind that contention would improve this answer. \$\endgroup\$ – Vigil May 2 at 12:25
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I would say that the Zone is rendered useless as it is a spell used to gather information and affects the mind (page 260 PHB).

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  • \$\begingroup\$ Could you support this answer by citing evidence and explaining your logic? \$\endgroup\$ – V2Blast Apr 27 at 7:16

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