Brain Dump. Whenever the neh-thalggu consumes a brain, it gains the magical ability to speak and understand languages known by the brain's previous owner.

Unusual Nature. The neh-thalggu doesn't require air.

Spellcasting (Psionics). The neh-thalggu casts one of the following spells, requiring no spell components and using Intelligence as the spellcasting ability (spell save DC 11). It must have consumed the requisite number of brains to cast the spell, as indicated:

1/day each: arms of Hadar (1 brain), detect magic (2 brains), magic missile (3 brains), Tenser's floating disk (4 brains), darkness (5 brains), hold person (6 brains), invisibility (7 brains), spider climb (8 brains), fear (9 brains), hypnotic pattern (10 brains), major image (11 brains), stinking cloud (12 brains)

Multiattack. The neh-thalggu makes one Bite attack and two Claw attacks.

Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit:11 (2d8 + 2) piercing damage.

Claw. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit:4 (1d4 + 2) slashing damage.

Extract Brain. Melee Weapon Attack: +4 to hit, reach 5 ft., one incapacitated Humanoid. Hit:35 (10d6) piercing damage. If this damage reduces the target to 0 hit points, the neh-thalggu kills the target by extracting and consuming its brain.

Mind Blast (Recharge 5–6). The neh-thalggu magically emits psychic energy at one Humanoid it can see within 10 feet of itself. The target must make a DC 14 Wisdom saving throw. On a failed save, the target takes 9 (2d8) psychic damage and is incapacitated until the end of its next turn. On a successful save, the target takes half as much damage and isn't incapacitated.