Can an Intellect Devourer mentally detect a Flumph?

Question

This is a follow-up to Would Truesight detect an Intellect Devourer?. Basically, one of my NPCs in the adventure I'm writing is secretly occupied by an Intellect Devourer, and I'm sorting out what her capabilities and limitations should be. I'm considering having a Flumph dwelling in her abode, who might be able to aid the party. Now I'm trying to figure out if the Flumph would survive for very long there.

The Intellect Devourer's Detect Sentience ability is only thwarted by a Mind Blank spell, while the Flumph's Telepathic Shroud is kind of a subset of the Mind Blank spell description. By the "letter of the law" she should know exactly where the Flumph is within 300', which in all likelihood means a dead or driven-off flumph.

I'm leaning towards an interpretation that the Intellect Devourer's ability only partially works- (she knows the Flumph is around, but not precisely where), but if there's any guidance or rules clarifications out there, I'd love to hear it.

Answer 1

Detect Sentience. The intellect devourer can sense the presence and location of any creature within 300 feet of it that has an Intelligence of 3 or higher, regardless of interposing barriers, unless the creature is protected by a mind blank spell. (MM 191)

Telepathic Shroud. The flumph is immune to any effect that would sense its emotions or read its thoughts, as well as all divination spells. (MM 135)

(all emphasis mine)

Looking at the abilities in question the intellect devourer should have no problem sensing the flumph. Neither location or position is an emotion or thought and the ability is not (nor does it replicate) a divination spell.

However as the GM you are free to decide otherwise. Looking at the flavor of the abilities a case can be made that the flumph is protected. Or you can just make it an elder flumph that has improved telepathic shroud.

Answer 2

Mindblank
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.

Comparing the telepathic shroud with the mindblank text gives us an easy comparison.

• Is the intellect devourer's detection ability psychic damage or the charmed condition? Nope
• Is it an effect that would sense its emotions or read its thoughts? Not as written
• Is the intellect devourer's detection ability a divination spell? It is similar, but also distinctly different
• Is it an effect that would allow the intellect devourer to gain information about the target? Yep

The obvious yes answer there is an ability that isn't included in telepathic shroud. That leaves us with a maybe where the mindblank spell and thought shroud overlap. I think that is obviously where your question is directed. We can get some help from a divination spell:

Detect Thoughts
... You can also use this spell to detect the presence of thinking creatures you can't see. When you cast the spell or as your action during the duration, you can search for thoughts within 30 feet of you. The spell can penetrate barriers, but 2 feet of rock, 2 inches of any metal other than lead, or a thin sheet of lead blocks you. You can't detect a creature with an Intelligence of 3 or lower or one that doesn't speak any language.

A notable difference here is that detect thoughts doesn't work on creatures that don't speak any language, while the intellect devourer doesn't require any language, only the Intelligence of 3.

So what does that mean?

RAW: it looks like the intellect devourer could find the flumph given that the thought shroud's explicitly defined protections don't cover the intellect devourer's detection ability and the mindblank spell includes an additional clause with an ability that does.

RAI: it looks like the flumph might me safe. The intellect devourer's ability is very similar to a divination spell, and it appears to be the intent of thought shroud to block attempts to locate it using divination effects. Additionally the intellect devourer's ability is called detect sentience, and sentience is the capacity to feel, perceive, or experience subjectively. That feel component is important because it could be taken to mean "the conscious subjective experience of emotion." All that wikipedia digging to say detecting sentience is detecting the capability to experience emotions, while thought shield includes protections from emotional detection.