Legendary Resistance (At Will). If Acrimony fails a saving throw, he can choose to succeed instead.

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

Hellish Gaurd. Any time Acrimony is targeted by a magic missile spell, a line spell, or a spell that requires a ranged attack roll, roll a d6. On a 1 to 5, Acrimony is unaffected. On a 6, Acrimony is unaffected, and the effect is reflected back at the caster as though it originated from Acrimony, turning the caster into the target.

Hellish Vengeance. Acrimony deals double damage against creatures that have damaged him previously.

Multiattack. Each creature of Acrimony's choice that are within his sight must succeed on a DC 45 Wisdom saving throw or become frightened permanently. Once active, this condition cannot subside or be prevented. Acrimony then makes five attacks, choosing from the ones listed below: 

Agonizing Halt. Acrimony magically emits vitality in a 500-foot cone. Each creature in that area must succeed on a DC 45 Intelligence saving throw or take 80 (8d10) force damage and be stunned permanently. A creature can repeat the saving throw at the end of each of their turns, ending themselves being stunned on a success.

Squish. Acrimony steps on his enemies. Up to ten creatures within 500 feet of Acrimony must make a dexterity saving throw, DC 45, or take a 500 bludgeoning damage. On a successful save the creature gains two exhausted levels.

Horns. Melee Weapon Attack: +59 to hit, reach 1000ft., one target. Hit: 700 (70d10) piercing damage.

Infernous Blade. Melee Weapon Attack: +59 to hit, reach 1000ft., one target. Hit: 700 (70d10) fire & force damage. If the target is a creature, it must succeed on a DC 45 Strength saving throw or be knocked prone.

Swallow. Acrimony swallows a creature he is holding. While swallowed, the creature is blinded and restrained, they have total cover against attacks and other effects outside Acroimony, and takes 300 (50d10) bludgeoning damage at the start another creature's turns in combat. If a swallowed creature somehow escapes Acrimony, they will fall with prone in a space within 5 feet of the Acrimony. 

Blazing Rebuke. If a creature regains hitpoints while Acrimony can see them, have them make a con save, DC 45. If the creature fails then Acrimony gains a number of hit points equal to what they just gained

Hellish Ward. Acrimony interrupts a creature within 1000 feet of him who is in the process of casting a spell. The creature must make a dexterity saving throw, 30. If the creature fails and is casting a spell of 3rd level or lower, their spell fails and has no effect. If they are casting a spell of 4th level or higher, their spell fails and has no effect and they take 10d10 force damage

Acrimony 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. Acrminoy regains spent legendary actions at the start of his turn.

Attack. Acrimony makes one attack from the ones listed in his actions.

Move. Acrimony moves up to his full speed.

Nightmarish Meal (Costs 2 Actions). Acrimony makes a contested strength check on a creature within 800 feet of him. If successful, he performs the swallow action on them.