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

Magic Resistance. The Demon Beast of the Mist has advantage on saving throws against spells and other magical effects.

Berserk. Whenever the Demon Beast of the Mist starts its turn with 60 hit points or fewer, the Demon Beast of the Mist goes berserk. when the Berserk is activated an effect similar to mirror image happens until the whale willingly ends it or is unconscious. The Demon Beast of the Mist can also use her Mist of Mental Contamination ability, her slam and bite get an additional 3d6 damage and crit on a 19 too.

Siege Monster. The Demon Beast of the Mist deals double damage to objects and structures.

Multiattack. The Demon Beast of the Mist can use its Mist (either one). It then makes 2 attacks: one with its bite, and one slam attack. It can use its Swallow instead of its bite.

Bite. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 31 (6d6 + 10) piercing damage. If the target is a creature, it is grappled (escape DC 19). Until this grapple ends, the target is restrained, and the Demon Beast of the Mist can't bite another target.

Slam. Melee Weapon Attack: +10 to hit, reach 20ft., one target. Hit: 31 (6d6 + 10) bludgeoning damage. If the target is a creature, it must succeed on a DC 19 Strength saving throw or be knocked prone.

Swallow. The Demon Beast of the Mist makes one bite attack against a Large or smaller creature it is grappling. If the attack hits, the target takes the bite's damage, the target is swallowed, and the grapple ends. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside the Demon Beast of the Mist, and it takes 27 (5d10) acid damage at the start of each of the Demon Beast of the Mist's turns.

If the Demon Beast of the Mist takes 30 damage or more on a single turn from a creature inside it, the demon Beast of the Mist must succeed on a DC 20 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the demon Beast of the Mist. If the Demon Beast of the Mist dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 30 feet of movement, exiting prone.

Mist of Elimination (Recharge 5–6). The Demon Beast of the Mist exudes a thick white fog that covers a 300 ft radius. The area is Heavily obscured It lasts until a wind of moderate or greater speed (at least 10 miles per hour) disperses it. Any creature that dies within the fog can't be brought back to life.

Mist of Mental Contamination [Berserk only] (Recharge 5–6). The Demon Beast of the Mist exudes a thick white fog that covers a 120 ft radius. Any creature in the area must succeed a DC 19 Constitution saving throw, taking 67 (15d8) acid damage on a failed save, or half as much damage on a successful one.

The Demon Beast of the Mist 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 Demon Beast of the Mist regains spent legendary actions at the start of its turn.

Move. The Demon Beast of the Mist moves up to half its speed.

Slam Attack. The Demon Beast of the Mist makes a slam attack.

Scream (Costs 2 Actions). The Demon Beast of the Mist screeches a loud, threatening roar. Each creature of the Demon Beast of the Mist's choice within 120 feet of it and aware of it must succeed on a DC 19 Wisdom saving throw or take 40 (8d6 + 12) psychic damage and become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, with disadvantage if the Demon Beast of the Mist is within line of sight, 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 Demon Beast of the Mist's Scream for the next 24 hours.