Avatar of Abhorrence (Aberrant Form)

Call of Carceri. A creature that is reduced to 0 hit points while within 300 ft of the avatar has disadvantage on death saving throws until it regains at least 1 hit point, and if it dies its soul is immediately transported to Carceri.

Unusual Nature. The avatar doesn’t require air, food, drink, or sleep.

Melded Presence. The avatar is two beings melded into one. It may shift between its human form and its aberrant form at the beginning of each of its turns (requires no action) or as a legendary action. When it does so, its features and abilities change.

Loathsome Aura. Any non-aberration creature that starts its turn within 20 feet of the Avatar must succeed on a DC 18 Constitution saving throw or take 14 (4d6) necrotic damage. Creatures inside of this area cannot regain hit points from spells.

Devouring Ground. The ground where the avatar walks becomes difficult terrain to all creatures except the Avatar. Creatures inside of this area have disadvantage on their Dexterity saving throws. These areas last until targeted by a spell of 4th- level or higher that deals radiant damage, such as the sunbeam spell.

Multiattack. The avatar makes two Enervating Focus attacks. One of which can be replaced by Sentence to Carceri, if available.

Enervating Focus. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 28 (5d8 + 6) necrotic damage. The target must succeed on a DC 19 Constitution saving throw or its hit point maximum is reduced by an amount equal to half the necrotic damage taken, rounded down. This reduction lasts until the target finishes a long rest or is targeted by a greater restoration spell. The target dies if its hit point maximum is reduced to 0.

Sentence to Carceri (Recharge 5-6). The avatar points at one creature that it can see within 300 feet of it. The target must succeed on a DC 20 Dexterity saving throw or take 16 (3d8+4) necrotic damage and become trapped inside a cage of tar that slowly begins to sink downwards. The cage is 10 ft tall and moves downwards with a speed of 5 ft/ round. It causes the ground in a 10 ft radius centered on the cage to become Devouring Ground should it touch it. A creature submerged in the tar cannot breathe and take 16 (3d8+4) necrotic damage at the start of each of their turns. 

The cage has an AC of 21, 60 Hit points and is resistant to all magical damage except radiant. It is immune to piercing, slashing and bludgeoning damage from nonmagical attacks and its movement speed cannot be altered.

Abhorrent Stride (2 Actions). The Avatar moves up to its speed without provoking opportunity attacks. All creatures whose space it moves through have to make a Dexterity 17 saving throw or take 8 (2d6+2) bludgeoning damage and be knocked prone.

Shift (2 Actions). The avatar shifts into its human form.

Drown in Tar (3 Actions). The avatar targets a creature currently inside of its Devouring Ground. That creature has to make a Strength 18 saving throw or become pulled under as a thick chain of tar wraps around their feet, taking 16 (3d8+4) necrotic damage at the start of each of its turns while submerged. Another creature may use its action to make a DC 18 Athletics (Strength) check to attempt to pull a submerged creature out.

The chain has an AC of 18, 20 Hit points and is resistant to all damage except radiant. A submerged creature sees their own chain. If it breaks, the creature may use half its movement to pull themselves to the surface.