Bloody Pursuit. The Abomination of Slaughter's spellcasting ability is Charisma (spell save DC 22). It can cast plane shift once a day, replacing the material component with freshly spilled blood of a creature native to the destination plane.

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

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

Life Consumption. If a creature is reduced to 0 hit points while grappled by the Abomination of Slaughter, it instantly dies and it's body is absorbed into the Abomination's mass. Creatures killed in this way cannot be resurrected until the Abomination of Slaughter is killed.

Regeneration. The Abomination of Slaughter regains 10 hit points at the start of its turn if it has at least one hit point.

Multiattack. The Abomination of Slaughter uses frightful presence, then makes one tail attack and two claw attacks.

Claw. Melee Weapon Attack: +14 to hit, reach 10ft., one target. Hit: 26 (3d12 + 7) slashing damage. 

Tail. Melee Weapon Attack: +14 to hit, reach 20ft., one target. Hit: 20 (3d8 + 7) bludgeoning damage, and it is grappled (escape DC: 22). Until this grapple ends, the target is restrained. The tail can hold only one target at a time.

Frightful Presence. Each creature of the Abomination of Slaugher's choice that is within 120 feet of the Abomination of Slaugher and aware of it must succeed on a DC 22 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the Abomination of Slaugher's Frightful Presence for the next 24 hours.

Blood Gout. When the Abomination of Slaughter takes damage, it releases a disproportionate amount of blood. The area within 10ft of it becomes difficult terrain to other creatures, and any other creature that starts its turn in the area takes 11 (2d10) acid damage. The blood remains caustic for up to 24 hours after being spilled.

The Abomination of Slaughter can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The Abomination of Slaughter regains spent legendary actions at the start of its turn.

Violent Advance. If the creature who just ended its turn dealt damage on its turn, the Abomination of Slaughter moves up to 30ft closer to that creature and makes a claw attack against it if able.

Gush. If the Abomination of Slaughter is currently standing in a pool of blood created by its blood gout ability, the pool increases its radius by 10ft.

Lost Soldiers (Costs 3 actions). A horde of spectral spirits of absorbed souls surround the Abomination of Slaughter. Until the start of its next turn, attacks made against the Abomination of Slaughter have disadvantage, and creatures within 30ft of the Abomination of Slaughter that damage the Abomination of Slaughter immediately take damage equal to half the damage dealt.