Amphibious. The xerfilstyx can breathe air and water. Devil's Sight. Magical darkness doesn't impede the devil's darkvision.

Fear Aura. A creature that starts its turn within 20 feet of the xerfilstyx must succeed on a DC 17 Wisdom saving throw, unless the xerfilstyx is incapacitated. On a failed save, the creature is frightened for 1 minute. While frightened in this way, a creature must take the Dash action and move away from the xerfilstyx by the safest available route on each of its turns, unless there is nowhere to move, in which case it needn't take the Dash action. If the creature ends its turn in a location where it doesn't have line of sight to the xerfilstyx, the creature can repeat the saving throw. On a success, the effect ends If a creature's saving throw is successful, the creature is immune to the xerfilstyx's Fear Aura for the next 24 hours.

Innate Spellcasting. The xerfilstyx's spellcasting ability is Charisma (spell save DC 17,  +9 to hit with spell attacks). It can innately cast the following spells, requiring no material components:

At will: detect evil and good, detect magic, ray of frost (3d8)

3/day each: charm person, dispel magic, major image, suggestion

1/day each: unholy aura

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

Spell Steal. As a bonus action, the xerfilstyx can steal a spell prepared by a creature it is grappling with. The target must make a DC 17 Wisdom saving throw or lose one of its spells (chosen at random) as if the creature had already cast it. The stolen spell is added to the xerfilstyx's list of spells that it can cast 1/day. The xerfilstyx loses all spells acquired this way after 24 hours.

Multiattack. The xerfilstyx makes three attacks: two with its greatsword and one with its tail.

Greatsword. Melee Weapon Attack: +11 to hit, reach 15 ft., one target. Hit: 20 (4d6+ 6) slashing damage.

Tail. Melee Weapon Attack: +11 to hit, reach 20 ft., one target. Hit: 10 (1d8 + 6) bludgeoning damage and the target must make a DC 16 Strength saving throw. On a failed save, the target is grappled (escape DC 16) and pulled to a space within 5 feet of the xerfilstyx. While grappled this way, the target is restrained, takes 18 (4d8) necrotic damage at the start of each of the xerfilstyx's turns, and the xerfilstyx can't use its tail attack against other targets.

Blood Breath (Recharge after a Long Rest). The xerfilstyx exhales scalding blood in a 60-foot cone. Each creature in that area must succeed on a DC 19 Constitution saving throw, taking 22 (5d8) fire damage and 22 (5d8) necrotic damage on a failed save, or half as much damage on a successful one.

Teleport (1/Day). The xerfilstyx magically teleports, along with any equipment it is wearing and carrying, up to 120 feet to an unoccupied space it can see.