Can an intellect devourer use its body thief ability on the undead? Relevant bits:
-- I'm pretty sure you can perform a coup de grace on undead, but their immunity means they will not have to make the fort save; they just take the crit damage which might or might not destroy them.
-- The body thief ability says: "If the victim is slain (or already dead), the intellect devourer usurps control of the body and may use it as its own, as if it controlled the target via a dominate monster spell."
This is some extremely gray area, and I'm tempted to say "it just works" in my game. However, the clause "as if...via a dominate monster" makes me wonder if this is a mind-affecting effect, which would make undead immune. Any thoughts?
I think the result mind be the intellect devour residing inside the still in-control undead since it is immune to mind-affecting.
I guess I would say it ceases to be an Undead the moment you kill it (again) and just becomes a corpse of the appropriate type.
My understanding is that once an undead is killed, again, it is in a state of destroyed, it is not dead, it is for all intents and purposes an object. It's why you can not raise dead on an undead. I would rule that an intellect devoured can not subsume an undead. I know no RAW to back me up though.
All we really have in this sort of situation is anecdotal... references to "destruction" representing "disruption of the magic animating it", and other fluff descriptions. The challenge is that to fit it into the rules, "Undead" is a creature type, when in reality it is more of a state of existence for an animated corpse.
I think this one really just requires GM Adjudication... and really, with the other issues going on I don't think it warrants a FAQ given how many other outstanding, more pertinent and common issues, exist.... so I definitely won't be flagging it such.