This was an interesting conversion since Intellect Devourers in 5e would be no challenge against a party of 12th level characters. I ended up looking at the Pathfinder version as well as all the 4e versions to get ideas on the best way to create a challenge. I wasn’t really happy with any of them so I ended up modifying the original 5e Intellect Devourer who has some nasty abilities when you increase the DCs.
Armour Class 17 (psychic defence)
Hit Points 237 (79d4 + 79)
Speed 40 ft.
STR 6 (+5) DEX 14 (+2) CON 13 (+1)
INT 12 (+1) WIS 11 (+0) CHA 22 (+6)
Skills Perception +6, Stealth +8
Damage Resistances cold, lighting, thunder; bludgeoning, piercing, and slashing from non-magical weapons
Condition Immunities blinded, fire, psychic
Senses blindsight 60ft. (blind beyond this radius), passive Perception 16
Languages understands Deep Speech and Aklo but can ‘t speak, telepathy 120ft.
Challenge 12 (8.400 XP)
Detect Sentience. The intellect devourer can sense the presence and location of any creature within 300 feet of it that has an Intelligence of 3 or higher, regardless of interposing barriers, unless the creature is protected by a mind blank spell or similar psionic affect.
Maukui can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. Maukui regains spent legendary actions after a long rest. It may use these actions in addition to any of its host body. These are psionic abilities with a DC of 18 (Charisma is the casting ability).
- Dimension Door. Teleport 500 feet in any direction.
- Mirror Image. Three duplicates appear. Roll a d20 to divert an attack to a duplicate – if three, 6+, four 8+ and 1 is 11+. Duplicate AC is 12.
- Web. Each creature that starts its turn in the webs or that enters them during its turn must make a Dexterity saving throw. On a failed save, the creature is restrained as long as it remains in the webs or until it breaks free. A creature restrained by the webs can use its action to make a Strength check against your spell save De. If it succeeds, it is no longer restrained. The webs are flammable. Any 5-foot cube of webs exposed to fire burns away in 1 round, dealing 2d4 fire damage to any creature that starts its turn in the /ire.
- Hold Person. Paralysed for 1 minute on a failed Wisdom saving throw.
- Spiked Pit. Summoning an extra dimensional pit up to 50 feet deep filled with spikes that lasts 1 minute. Creatures must make a Dexterity saving throw to avoid falling. Those that fail take 15 (5d6) bludgeoning damage from the fall and 6 (2d6) piercing damage from the spikes at the bottom of the pit. The climb DC is 20 but even with success the spikes do 3 (1d6) piercing damage per round.
Multiattack. Maukui makes four attacks with its claws and uses Devour Intellect. It may also use Thought Lance when available.
Claws. Melee Weapon Attack: +8 to hit, reach 5 ft., one target.
Hit: 7 (2d4 + 2) slashing damage.
Devour Intellect. Maukui targets one creature it can see within 10 feet of it that has a brain. The target must succeed on a DC 17 Intelligence saving throw against this magic or take 11 (2d10) psychic damage. Also on a failure, roll 3d6: If the total equals or exceeds the target’s Intelligence score, that score is reduced to 0. The target is incapacitated until it regains at least one point of Intelligence.
Body Thief. Maukui initiates an Intelligence contest with an incapacitated creature within 5 feet of it. If it wins the contest, 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 outs ide its host. The intellect devourer retains its Intelligence, Wisdom, and Charisma scores, as well l 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 the host body drops to 0 hit points, the intellect devourer must leave it. A protection from evil cast on the body drives the intellect devourer out. The intellect devourer is also forced out if the target regains its devoured brain by means of a wish. By spending 5 feet of its movement, the intellect devourer can voluntarily leave the body, teleporting to the nearest unoccupied space within 5 feet of it. The body then dies, unless its brain is restored within 1 round.
Thought Lance (Recharge 5-6). Maukui psionically emits psychic energy in a 60-foot cone. Each creature in that area must succeed on a DC 17 Intelligence saving throw or take 54 (12d8) psychic damage and gain vulnerability to psychic damage for 1 minute. On a successful save the creature takes half-damage and is not vulnerable.
Devoid of a head, or any features at all save for four short, clawed legs, this creature’s body looks like a large, glistening brain.