Tembril

Ambusher. In the first round of combat, the tembril has advantage on attack rolls against any creature it has surprised.

Nimble Leap. The tembril can take the Dash or Disengage action as a bonus action on each of its turns.

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.