An Intellect Devourer Has an ability called Body Thief, which states:
…the intellect devourer magically consumes
the target’s brain, teleports into the target’s skull, and takes
control of the target’s body. While inside a creature, the
intellect devourer has total cover against attacks and other
effects originating outside its host. The intellect devourer
retains its Intelligence, Wisdom, and Charisma scores, as
well as its understanding of Deep Speech, its telepathy, and
its traits. It otherwise adopts the target’s statistics. It knows
everything the creature knew, including spells and languages.
If an Intellect Devourer takes over a Simulacrum of a Wizard, can the Intellect Devourer recover spells with a long rest, or is it beholden to the simulacrum rule:
The simulacrum lacks the ability to learn or become more powerful, so it never increases its level or other abilities, nor can it regain expended spell slots.
This literally just happened to us and we are stumped. I think the Devourer is a new creature and thus can recover spells. As it would choose to gain the abilities but not the short comings of the new form.
Best Answer
The problem is in trying to apply Body Thief to the Simulacrum in the first place. The RAW does not support that.
The Body Thief Action for intellect devourer says:
So if the Simulacrum spell creates a creature of type humanoid, it could be targeted by this action. But let's see what the spell description says:
So it turns out, it is a construct, the resemblance of which to a beast or humanoid is "illusory". So, though it is a creature, the type of creature it is, is other than humanoid, and thus it cannot be the target of the Body Thief Action.