The last player character that died in the area has their soul joined with the monster. The player soul adds an additional 1A per turn, which is played out by the deceased PC. The creature can only hold one soul at a time. If another is devoured, the soul is replaced, and the previous returns to its body. The player is comatose but alive.

Demoralising Whispers. The Abomination knows your deepest insecurities and fears, telepathically suggesting a course of action to each player. While it can see any creature and isn't incapacitated, each creature that starts its turn within 30 feet must succeed on a DC 10 Wisdom saving throw.
On a failure, the character rolls a d8 to determine what it does during its turn. On a 1 to 4, the creature can take no actions, but can take bonus actions and move.
On a 5 or 6, the creature takes no action or bonus action and uses all its movement to move in a direction of the DMs choosing.
On a 7 or 8, the creature makes a melee or cantrip attack against a randomly determined creature within reach or does nothing if it can't make such an attack.

Slam. Melee Attack: +5 to hit, reach 5 ft., one creature. Hit: 8 bludgeoning damage. If the target is Medium or smaller, it must succeed on a DC 15 Strength saving throw or be knocked prone.

Ground Slam. Area Attack: Hits all creatures on ground, reach 5 ft all directions. 
On Hit: 6 bludgeoning damage. If the target is Medium or smaller, it must succeed on a DC 10 Strength saving throw or be knocked prone.

Projectile Soul Plasma (Charge 1, Recharge 2). Ranged Attack to Area.
On Initial Hit: 1d8 necrotic damage, Upon standing in: 1d4 necrotic damage.
Spits a straight line of 5ft wide glob in the direction its facing, up to 40 feet away. Must have charged this in the last turn and not moved since. Plasma stays on the floor for rest of fight.Each creature hit must succeed on a DC 13 Dexterity saving throw or be restrained until the end of the monster's next turn.

Soul Tether (Recharge 3).  Ranged Attack:
Monster chooses one character's soul to join with, from within 120ft. On the character's next turn, they must succeed on a DC 15 Charisma saving throw or be forced to spend all normal available movement to travel towards the monster. Player actions are unaffected. This process breaks when the character reaches the monster or succeeds a Charisma saving throw on the start of their turn.

Upon reaching 50% health, unleashes a high pitched shriek of wailing souls. Damages all characters with 50ft, that can hear, with 2 sonic damage