Faolyn
(she/her)
So far, very, very few LU monsters have multiattack. Even monsters that really should have them. I don't know why.However, this versions doesn't it all, it just has damage. Additionally, though I haven't run the numbers 14 DPR seems low for a CR 3 monster.
I think it at least needs the multiattack action and I would have liked to see a less istun-lock version of devour intellect.
Don't forget that the o5e intellect devourer basically causes infinite stun. I think there's a rule in one of the books that says that a lowered attribute is restored by 1 point for every three days or a week of rest, or something, but I'm having a really hard time finding that rule again. Beyond that, there's greater restoration, which requires a 9th-level caster--and you're still out of luck if you only have one caster in the party who can cast that, and that's the PC that gets its intellect devoured.
Which reminds me... @Morrus, the Body Thief ability is pretty vague here. It says that the ID eats the brain, but there's no damage listed. It also says that the ID will leave the body if the body is reduced to 0 hp--but eating the brain should cause the body to die. The text indicates that the body is dead.
May I suggest using the information for the dybbuk in Mordenkainan's as a template? In effect, the ID gains a number of temp hp equal to the body's hp max, retains its own mental attributes, saves, skills, resistances, and immunities, and uses the body's physical attributes. When the ID runs out of temp hp, it can no longer control the body and has to leave it.