Shapechanger. The Abomination can use its bonus action to polymorph into a Large demonic humanoid or back into its true form, which is humanoid. Its statistics, other than its size and AC, are the same in either form. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.

Ungodly strength. The Abomination has advantage on athletics checks

Multiattack. In demonic form, the abomination makes two rake attacks and one rage attack, if available. In humanoid form, it makes two manacle attacks.

Manacle (Humanoid Form Only). Melee Weapon Attack: +9 to hit, reach 15 ft., one target. Hit: 8 (1d4+ 6) bludgeoning damage. Once a turn a creature hit with this attack can be forced to make a DC 11 constitution saving throw or be stunned until the end of their next turn

Beasts bile (Humanoid Form Only) (Recharge 4-6). The abomination releases a horrid concoction of various fluids from their mouth. Every creature within a 15 foot cone must make a DC 16 constitution saving throw or take 17 (5d6) poison damage, taking half as much on a successful save

Absolution (Humanoid Form Only) (Recharge 6). The abomination kneels and forgives itself for the situation it is in, for it is not truly it's fault. It heals for 17 (5d6) and becomes immune to being stunned, poisoned, and incapacitated for the next minute, unless it transforms into its demonic form, in which case it loses any immunity

Rake (Demonic Form Only). Melee weapon attack: +9 to hit, reach 5 ft., two targets. Hit: 7 (1d6+6) slashing damage

Rage (Demonic Form Only) (Recharge 4-6). Melee weapon attack: +9 to hit, reach 5 ft., one target. Hit: 18 (3d6+6) slashing damage. A creature hit with this attack must make a DC 17 wisdom save or be frightened until the end of the abomination next turn.

Slam (Demonic form only) (Recharge 6). The abomination slams head first into a creature, breaking ribs and sending them flying. The creature must make a DC 17 dexterity saving throw or take 35 (10d6) bludgeoning damage and be sent back 25 ft..