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Let's assume that a new party member, already possessed by an intellect devourer, is true polymorphed into a dog by the wizard as part of his initiation ritual (and a big message that said "don't mess with me").

What would happen to the intellect devourer and/or the possessed member? Keep in mind

The intellect devourer initiates an Intelligence contest with an incapacitated humanoid

Also,

The intellect devourer is also forced out if the target regains its devoured brain by means of a wish.

If the new member regains its brain, the intellect devourer is forced out, but the wish part makes me think that spells like regenerate won't work on restoring the brain, since technically it's a "complete" creature, not a limb. And true polymorph doesn’t regenerate parts, it literally changes the creature into another creature or object.

But,

While inside a creature, the intellect devourer has total cover against attacks and other effects originating outside its host.

True polymorph originates from outside, but it affects everything inside and outside the target. Also, dispelling true polymorph reverts the creature to its original state with all its equipment. This might mean that the intellect devourer might be consider equipment and be absorbed, but there is this part where the intellect devourer is acting as its brain, giving the new member the status of a creature and maintaining the body alive (it is no different from a zombie, I guess).

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.

So, keeping it as RAW as possible, what happens? Is the intellect devourer treated as an object and absorbed and, if it's absorbed, does the original regain control while polymorphed? Is it expelled? Is it treated as a single entity therefore the true polymorphed creature is still the intellect devourer on a “new host”?

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Intellect devourer is not an object

It is an aberration of tiny size.

Nothing states, that they are one and the same entity.

Note, that it is possible to save the host. Furthermore rules state about Intellect devourer leaving the host, when he drops to 0 hp. The host and the Intellect Devourer are two seperate creatures.

How will polymorph work?

While inside a creature, the intellect devourer has total cover against attacks and other effects originating outside its host.

You cannot affect the intellect devourer itself, but only the host.

True polymorph

Choose one creature or nonmagical object that you can see within range.[...]

The only thing, you will see is once again, the host, not the intellect devourer. Accordingly, to the spell's decription, only one creature will be affected and that will be the host.

What will be the effect?

Despite the fact of being polymorphed, the host is still the same creature.

The effect will be a dog with the intellect devourer inside it's skull. Because True Polymorph usually affects mental ability scores it is possible to notice, that something is wrong, as the dog will have an Intelligence of 12, instead of 3.

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  • \$\begingroup\$ If is not treated as an object or as the same entity, would not the intellect devourer be considered as a creature touching the target and expell it? Also, the whole deal about true polymproph is that the target no longer is, while the spell last, the same creature. An that also apply for the humanoid part, is the intellect devourer be able to control the host since is not longer a humanoid? \$\endgroup\$ – Chepelink Aug 30 '16 at 12:00
  • \$\begingroup\$ @chepelink Doesn't the dog still die if the Intellect Devourer leaves the brain? The polymorphed creature is the same "person" but isn't a humanoid. The target’s game statistics, including mental ability scores, are replaced by the statistics of the new form. It retains its alignment and personality. \$\endgroup\$ – KorvinStarmast Aug 30 '16 at 12:38
  • \$\begingroup\$ @KorvinStarmast That depends on when it leaves/is expelled. If its at the moment polymorph is being cast, no, it won't die. If its after, that depends, if it is expelled because of a new brain (as in wish spell) no, it won't die, otherwise, yes, the dog will die after a round. I have the feeling that the best answer is going to be "it is entirely up to your DM" because of all the caveats and loopholes it has. \$\endgroup\$ – Chepelink Aug 30 '16 at 13:58
  • \$\begingroup\$ The "humanoid" restriction only applies to the original possession attack, not to remaining within the host. So despite the fact that an intellect devourer can't take over a regular dog, if it's within a creature that becomes a dog, the creature type doesn't matter. \$\endgroup\$ – Polisurgist Aug 30 '16 at 16:15
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    \$\begingroup\$ Can we take this even further? Can the Intellect Devourer inhabit the host if the host is polymorphed into tiny creature, say a pixie? \$\endgroup\$ – LegendaryDude Aug 30 '16 at 17:24
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Polymorph replaces your mental scores, so the Intellect Devourer will be forced out and the Polymorph spell will fail within 1 round.

Pertinent text from Polymorph, PHB pg.266 (emphasis mine):

The transformation lasts for the duration, or until the target drops to 0 hit points or dies. The new form can be any beast whose challenge rating is equal to or less than the target’s (or the target’s level, if it doesn't have a challenge rating). The target’s game statistics, including mental ability scores, are replaced by the statistics of the chosen beast. It retains its alignment and personality.

Since your mental scores are replaced, this means you have a new brain growing during the spell. Which means the Intellect Devourer will be forced out of the body since a new brain is growing in place and it doesn't have any room to occupy inside the skull.

So what will happen is that the Intellect Devourer will leave the body, in which case the caster doing the polymorph needs to spot the Intellect Devourer and recognize what has happened in order to cast polymorph again. If they don't, then the newly polymorphed creature's brain is destroyed, and it dies.

When this happens, the polymorphed creature reverts to it's previous state, which is now brainless since the Intellect Devourer ate their brain previously with Body Thief.

So then the character dies (I mean...... unless they're a Barbarian. :P ) Basically, the moral of this story is, don't get your brain eaten or you'll need a Wish spell to get it back. Alternately, a True Polymorph could permanently fix a member who has had their brain devoured, but if it were ever dispelled they would die.

You know, for a CR 2, Intellect Devourer's are pure evil.

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  • \$\begingroup\$ The problem that I have with 5e is that the way creatures are "defined" (a corpse is an object, a corpse with a tiny bit of magic is a creature). Unless something happen in the first round after the ID is expelled, the host is basically a corpse (an object). This is: Host + ID = Creature with the physical attributes of the host and the mental attributes of the ID. When polymorphing a creature you substitute the mental attributes of the creature, and these are given by the ID, not by the original host. Thus, would it "grow a brain" or would it "polymorph" the ID as if it were the brain? \$\endgroup\$ – Chepelink Sep 16 '16 at 11:52
  • \$\begingroup\$ Well since the character can be restored, they aren't an object because they aren't actually dead yet. The Intellect Devourer ability doesn't say that it changes your character into an object, so you're still a player, but under control of something else. This isn't all that different from being possessed by a ghost, except for the incredible lethality attached to this. I would, as the DM, allow for the player to be revived, especially since the ID is a CR 2. \$\endgroup\$ – Lino Frank Ciaralli Sep 16 '16 at 16:34

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