Does the Protection from Evil and Good spell kill the host of an Intellect Devourer?

Question

The Body Thief action in the Intellect Devourer's statblock in the MM and SRD says, in part:

A protection from evil and good spell cast on the body drives the intellect devourer out. [...] By spending 5 feet of its movement, the intellect devourer can voluntarily leave the body, teleporting to the nearest unoccupied space within 5 feet of it. The body then dies, unless its brain is restored within 1 round.

Is a character functionally dead immediately upon losing an Intelligence contest vs. an Intellect Devourer (barring a wish spell)?

It seems rather draconian for a CR2 creature, if so. (Hell, even if not so; requiring one of those two specific spells seems rough even if one is a 1st-level spell - that is, assuming it even helps.)

Answer 1

The intellect devourer consumes its host's mind in a two step process

First it uses "Devour Intellect"

The intellect devourer targets one creature it can see within 10 feet of it that has a brain. The target must succeed on a DC 12 Intelligence saving throw against this magic or take 11 (2d10) psychic damage. Also on a failure, roll 3d6: If the total equals or exceeds the target’s Intelligence score, that score is reduced to 0. The target is stunned until it regains at least one point of Intelligence

This ability does not kill the potential host - but it does drop its intelligence to 0 and stun it. This is powerful indeed, but look what the Devourer has to do to accomplish it: have the host fail a save, and then another mechanic which will only go in favor of the Devour half the time against a target of average intelligence. Effectively two failed saves on the same turn. And to do this, it has to be within 10 feet of its host, when it has an AC of 12 and 21 hp (although it does have non-magic damage resistance).

Once the host is stunned (and thus, incapacitated) and has an intelligence of 0 (and thus, -5 on its Intelligence contest) the Devourer can employ its second ability, "Body Thief"

The intellect devourer initiates an Intelligence contest with an incapacitated humanoid within 5 feet of it. If it wins the contest, the intellect devourer magically consumes the target’s brain, teleports into the target’s skull, and takes control of the target’s body.

This second ability does kill the host's mind, but it is unlikely to succeed unless the first ability has been used. Remember, to use this, the target must already be incapacitated. The Devourer has an Int mod of +1. Unless its potential host has also had its Int reduced, it doesn't have a great chance to win the Intelligence contest.

Although the Devourer has Multiattack, it cannot use both Devour Intellect and Body Thief on the same turn. So we are talking about a two round process requiring three successful rolls and unable to be initiated unless the Devourer is within 10 feet. This is dangerous against a single opponent who the Devourer manages to surprise. Against an alert party, even one of Level 2, the Devourer is unlikely to be able to finish the second ability, the one that actually kills the host.

Answer 2

Yes, the host dies

The second paragraph of the Body Thief ability describes all the ways in which the intellect devourer can leave the host body, then says that the host dies. It doesn't matter whether the intellect devourer left the body due to protection from evil and good or voluntarily - the brain is gone, so the body will die.

Used in this manner, protection from evil and good is offensive rather than defensive, depriving the intellect devourer of the abilities and guise of its host. Saving the host requires resurrection, reincarnate, or better.

Indeed, the target is as good as dead as soon as they succumb to Body Thief, although they are not literally dead until after the intellect devourer leaves.

This may seem rough, but note that Body Thief can only be used on an incapacitated target. The intellect devourer's best way to achieve this is through application of Devour Intellect or waiting for the target to sleep, although other creatures in an encounter may provide other ways. While the intellect devourer is particularly deadly beyond its CR (since resistance to Devour Intellect doesn't improve with level for most characters), in general being incapacitated in the presence of enemies is already deadly.