How does the spell Death Ward (DW) interact with the Intellect Devourer's (ID) "Body Thief" action? My gut instinct is that, assuming the ID is ejected by Protection from Evil and Good and not by dropping to zero HP–which would trigger the Death Ward early–it restores the (0 INT) brain immediately following expulsion, thus "negating the effect that would cause death."
A second reading would be that the instant death after ID-ejection is negated by DW the first round, but the character dies one round later unless the brain is restored by another means. This requires a bit of work, since there is technically no RAW that says you die if you have no brain for a round. Rather, the RAW state that you die one round after an ID which uses Body Thief vacates your skull.
Paradoxically, another good reading to me would be that it negates the death from having no brain for one round, after which the character lives a brainless, Stunned life until Greater Restoration is cast on them. (Preposterous, but a fair reading of RAW in my mind.)
A last reading I would add for completion is that DW could negate the Protection from Evil and Good spell as it would cause the ID to vacate the skull, killing the target one turn later. This doesn't seem likely to me, though, as this effect causes a conditional death. If conditional deaths were negated by DW as a matter of course, it would absorb effects before saving throws were rolled rather than after they were failed, which is obviously not how DW is used.
This same last argument is why I suspect DW won't simply block Body Thief overall, although I'm aware some folks rule that to be the simple case.
How does this interaction work?
Best Answer
Intellect Devourer magically consumes the brain of the victim. Once it leaves, there's just an empty hole where the brain used to be.
To quote the monster description:
Death Ward, on the other hand, prevents the subject from dropping to 0 HP - it drops to 1 HP instead. So while under DW, the Intellect Devourer is actually protected from ejection, if it indeed gets inside in the first place.
Once it leaves the brain has to be grown back in one round, or the subject dies.
I was about to write how DW would not protect the victim from dying because of a lack of a brain, but then it struck me that as a DM, I would probably rule that "magically removing someone's brain" would fall into the "effect that would kill it instantly" -category which DW protects the subject from.
Mostly because, while the body is still living under the Intellect Devourer's control, there's nothing of the victim left; no memories, intellect, etc.
(Which opens a whole new can of worms in the form of "brain vs soul" -discussion which is open to many interpretations...)
So how DW works in this case (without finding any official ruling on the matter saying otherwise), in my opinion, would be that the Intellect Devourer could not remove a target's brain and enter it, if the target was under DW's protection (which would then end DW and leave the target open for another try, later on).