After thinking about uses for Mind Blank and how it interacts with a bunch of spells, I then tried to think of counters and ways someone could get past it with one problem always coming up. How do you know when someone is under its effect?
Mind Blank spell description for reference:
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.
It seems like since it blocks divination spells and is powerful enough to foil a wish spell, a simple Detect Magic which is a divination spell would also not work for finding it. If that is the case, then is it even possible to detect someone under the affect of Mind Blank?
Best Answer
Ask them
Are you now, or have you ever been, a user of mind blank?
Interestingly, the spell zone of truth isn't divination - it's enchantment and doesn't seem to be affected by mind blank. This puts it an interesting position where it doesn't fall under the divination umbrella - and it also isn't giving information about the target. It is instead interpreting what they are saying: an external response analysis.
Once they're in the zone, ask them directly if they are under the effects of the spell mind blank. And yes, it's still possible for a sufficiently
weasellyclever DM to come up with a non-lie response that doesn't say, but it's a pretty solid path to finding out.This is a fairly straightforward and very unsubtle method, so its actual effectiveness may be pretty limited.