Keen Smell. The troll has advantage on Wisdom (Perception) checks that rely on smell.

Magic Resistance. The troll has advantage on saving throws against spells and other magical effect

Regeneration. The troll regains 10 hit points at the start of its turn. If the troll takes acid or fire damage, this trait doesn't function at the start of the troll's next turn. The troll dies only if it starts its turn with 0 hit points and doesn't regenerate.

Psionic Overwhelm. Each creature that starts its turn within 30 feet of the troll and can hear it must succeed on a DC 18 Intelligence saving throw. On a failed save the creature takes 19 (3d12) psychic damage and is stunned for 1 minute, on a successful save the creature takes half as much damage and is not stunned. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Multiattack. The troll makes three attacks: one with its bite and two with its claws.

Bite. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 7 (1d6 + 4) piercing damage.

Claw. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 11 (2d6 + 4) slashing damage.

Tentacles. Melee Weapon Attack: +9 to hit, reach 5 ft., one creature. Hit: 16 (3d10) psychic damage. If the target is Large or smaller, it is grappled (escape DC 18) and must succeed on a DC 18 Intelligence saving throw or be stunned until this grapple ends.

Extract Brain. Melee Weapon Attack: +9 to hit, reach 5 ft., one incapacitated creature grappled by the troll. Hit: The target takes 55 (10d10) piercing damage. If this damage reduces the target to 0 hit points, the troll kills the target by extracting and devouring its brain