Multiattack

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

Bite

Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 13 (2d8 + 4) piercing damage. If the tembril scores a critical hit against a Medium or smaller target and the damage reduces the target to below half its hp maximum, the target must succeed on a DC 15 Constitution saving throw or be instantly killed as its head is removed. A creature is immune to this effect if it is immune to piercing damage or doesn't have or need a head.

Claws

Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 11 (2d6 + 4) slashing damage.

Maddening Chitter (Recharge 6)

The tembril chitters maddeningly in a 30-foot cone. Each creature in that area must make a DC 15 Wisdom saving throw. On a failure, a creature takes 21 (6d6) psychic damage and is incapacitated for 1 minute. On a success, a creature takes half the damage and isn't incapacitated. If a creature fails the saving throw by 5 or more, it also suffers short-term madness. An incapacitated creature can repeat the saving throw at the end of each of its turns, ending the incapacitated condition on itself on a success.