Immutable Form. The horror is immune to any spell or effect that would alter its form.

Magic Resistance. The horror has advantage on saving throws against spells and other magical effects.

Multi-headed Fury. The horror has three heads, each capable of biting independently. When making a bite attack, the horror can use all three heads, dealing 3d6 + 2 piercing damage.

Multiattack. The horror makes three attacks: one with its bite and two with its choice of whip or claw.

Bite. Melee Weapon Attack (each head): +6 to hit, reach 10 ft., one target. Hit: 5  (1d6 + 2) piercing damage.

Whip. Melee Weapon Attack: +6 to hit, reach 15 ft., one target. Hit: 7 (1d10 + 2) slashing damage, and the target must succeed on a DC 15 Strength saving throw or be pulled up to 10 feet closer to the horror. If the target was already within 5 feet of the horror, it is instead grappled (escape DC 15).

Claw. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 8 (2d6 + 2) slashing damage.

Squealing Scream (Recharge 5–6). As a bonus action, the horror unleashes a piercing scream in a 30-foot cone. Each creature in that area must succeed on a DC 15 Constitution saving throw or be stunned until the end of its next turn and deafened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the deafened condition on a success.

Opportunity attack: Claw