Actions

Multiattack. Makes 1 Claw attacks and uses Devour Intellect.

Claws. Natural Weapon. +4 to hit.
Hit. 9 (2d6+2) Slashing damage.
Dispelling Bite. In lieu of another attack or Devour Intellect, as reaction after Claws hit. The creature can cause spells of Level 2 or lower to be dispelled, and can make an Intelligence check for higher level spells. Once a check fails, the effect ends.

Devour Intellect. The monster targets one creature it can see within 10 ft of it that has a brain. The target must succeed on a DC 12 INT save or take 11 (2d10) psychic damage.
Also on a failure, roll 3d6+2: If the total equals or exceeds the target's Intelligence, the parasite is teleported into the creature's brain and the host is subjected to either Dominate Person (L2) or Feeblemind (L2) until the parasite is removed.
The host body is stunned until it makes a DC 12 CON save as the parasite learns to operate its new body.
After taking over, the parasite knows everything the host knew, including spells, languages, and class features.
The host may attempt to reject the parasite up to three times by repeating the INT check as an action. However, after the third failure, they can no longer resist.
If the host body drops to 0 hit points, the parasite is teleported to a space within 5ft with 23hp.
The parasite may be forced out if the host is subjected to Psychic damage, forcing the parasite to make a CON save vs half the damage. By spending 20ft of its movement, the intellect devourer can voluntarily leave the body, teleporting to the nearest unoccupied space within 5 ft of it.

Life Drain. Melee Spell Attack: +4 to hit.
Hit. 11 (2d8+2) Psychic damage.
The target must succeed on a DC 12 INT Save or have its hit point maximum is reduced by the same.
This reduction lasts until the victim finishes a long rest.
If this effect reduces the victim's HP Max to 0, they fully die, and their corpse is cursed and cannot be resurrected while the parasite lives.