Legendary Resistance (3/Day). If the xerfilstyx fails a saving throw, it can choose to succeed instead.

Magic Resistance. The xerfilstyx has Advantage on saving throws against spells and other magical effects.

Watery Stealth. The xerfilstyx has Advantage on Dexterity (Stealth) checks made when trying to Hide in water, blood, or any other liquid.

Multiattack. The xerfilstyx makes one Rend attack, one Adhesive Tail attack, and one Gore attack.

Rend. Melee Weapon Attack: +11, reach 15 ft. Hit: 11 (2d4 + 6) Slashing damage plus 5 (2d4) Fire damage.

Adhesive Tail. Melee Weapon Attack: +11, reach 15 ft. Hit: 10 (1d8 + 6) Bludgeoning damage, and if the target is a Large or smaller creature it is also Grappled (escape DC 19) and pulled up to 15 feet toward the xerfilstyx.

Gore. Melee Attack Roll: +11, reach 10 ft. Hit: 10 (1d8 + 6) Piercing damage, and the target is pushed 5 feet away from the xerfilstyx and knocked Prone.

Boiling Blood (Recharge 5–6). The xerfilstyx spits a stream of blood from its mouth which sizzles on contact with flesh. Dexterity Saving Throw: DC 17, each creature in a 60-foot Cone. Failure: 58 (13d8) Fire damage. Success: Half damage.

Spellcasting. The xerfilstyx casts one of the following spells, requiring no Material components and using Charisma as the spellcasting ability (spell save DC 19):

At Will: Minor Illusion, Mage Hand, Detect Magic, Invisibility

1/Day: Cone of Cold, Hallow, Modify Memory

Drink Blood. Constitution Saving Throw: DC 19, one creature within 5 feet of the xerfilstyx that is Grappled. Failure: 5 (2d4) Necrotic damage, and the target's Hit Point Maximum is reduced by the same amount. The xerfilstyx regains Hit Points equal to the Necrotic damage dealt.

Legendary Action Uses: 3. Immediately after another creature’s turn, the xerfilstyx can expend a use to take one of the following actions. The xerfilstyx regains all expended uses at the start of each of its turns.

Frightening Presence. The xerfilstyx uses Spellcasting to cast Fear. This spell does not affect any Fiends, and the xerfilstyx can’t take this action again until the start of its next turn.

Control Foe. The xerfilstyx uses Spellcasting to cast Charm Person. The xerfilstyx can’t take this action again until the start of its next turn.

Advance. The xerfilstyx moves up to half its Speed, and it makes one Rend attack.