The Devour Intellect power doesn't make any sense at all. Reducing a character's Int to 0 should represent the complete destruction of their mind. The Intellect Devourer already has a different power that does exactly that. So why even have the Int reduction at all? It should just say "stunned" for whatever duration.
Also the flavor text on that page is dumb, but that's beside the point.
Why would Intelligence of 0 be the destruction of the mind? At least in 3.5, it was represnted by coma-like state, and in 5e it seems to be represented by the stunned condition.
The Devour Intellect action is written that way for a couple of reasons.
1. The stunned condition makes the target incapacitated. This allows body thief to be initiated. Otherwise, it would have to use its claws to knock someone unconscious.
2. The ability score reduction to 0 makes the Intelligence contest be in the intellect devourer's favor without the result being automatic.
For the OP:
As others have said, ability score damage is rare. In 3.5 ability score damage was recovered at a rate of 1 point per day. Ability score drain was permanent. 5e tosses all that out the window and relies on monster actions to describe the ability score loss (and possible recover). The spell
greater restoration acts as a general catchall for restoring an ability score. The shadow gives an indication of what ability score damage looks like in 5e --- the damage is recovered after a long rest (or
greater restoration). The intellect devourer shows how to handle ability score drain --- the drain is permanent until solved by magic such as
greater restoration. The wraith shows how to handle Constitution ability damage --- the maximum hit points are reduced instead.
What I usually do for creatures converted from 3.5 is to skip reduction in ability scores and instead give the affected creature disadvantage to all ability checks, saving throws, and attack rolls with the affected ability until the creature finishes a long rest or gets
greater restoration.
