Cataclysm's Leviathan

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

Partial Freeze. If the leviathan takes 50 cold damage or more during a single turn, the leviathan partially freezes; until the end of its next turn, its speeds are reduced to 20 feet, and it makes attack rolls with disadvantage.

Siege Monster. The leviathan deals double damage to objects and structures.

Water Form. The leviathan can enter a hostile creature’s space and stop there. It can move through a space as narrow as 1 inch wide without squeezing.

Abyssal Gravity. The area within 30 feet of the leviathan is difficult terrain. Creatures knocked prone in this area must spend all their movement to stand. Swimming creatures in this area have disadvantage on Dexterity saving throws.

The leviathan does not use Multiattack. It takes one action per turn, chosen from the options below.

Masticate. Melee Weapon Attack: +10 to hit, reach 20 ft., one target.
Hit:  8d6 piercing damage.

If the target is Large or smaller, it must succeed on a DC 16 Dexterity saving throw or be grappled (escape DC 16) and restrained.

If the target is already grappled by the leviathan, it takes an additional 4d10 piercing damage and is stunned until the end of its next turn.

Tentacle Torrent. Each creature in a 40-foot-radius, 180-degree arc must make a DC 16 Dexterity saving throw.

Failure: 6d10 bludgeoning damage, knocked prone, and pushed 25 feet.
Failure by 5 or more: The creature is also restrained until the end of its next turn.
Success: Half damage and not knocked prone.

Lingual Suppression. One creature within 40 feet must make a DC 18 Strength saving throw.

Failure: The creature is pulled adjacent to the leviathan and grappled (escape DC 16) and restrained.
Success: The creature is pulled 10 feet.

Frontal Rush. The leviathan charges up to 80 feet in a straight line.

Each creature in a 15-foot-wide line must make a DC 18 Dexterity saving throw.

Failure: 8d10 bludgeoning damage and knocked prone.
Failure by 5 or more: The creature is also stunned until the end of its next turn.
Success: Half damage and pushed aside.

The area becomes difficult terrain.

Abyssal Mines. The leviathan creates four abyssal mines within 60 feet.

A mine explodes when a creature enters its space or at the end of the round.

Creatures in a 15-foot radius must make a DC 16 Dexterity saving throw, taking 4d8 bludgeoning damage on a failure, or half on a success.

Disintegration Lock. One creature within 60 feet must make a DC 15 Constitution saving throw.

Failure: Restrained and takes 4d6 force damage at the start of each of its turns.
Success: Half damage and not restrained.

The effect lasts until the end of the leviathan’s next turn. Another creature can use an action to end the effect with a successful DC 16 Strength check.

The leviathan can take 3 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature’s turn.

Reposition. The leviathan moves up to 30 feet without provoking opportunity attacks.

Tail Jab. Melee Weapon Attack: +16 to hit, reach 15 ft., one target. Hit: 1 (3d8) bludgeoning damage.

Current Pull (Costs 2 Actions). Each creature within 20 feet must succeed on a DC 19 Strength saving throw or be pulled up to 15 feet toward the leviathan.

Exposed Heart.

When reduced to 0 hit points, the leviathan does not die. Its heart is exposed (AC 18, 150 hit points). While the heart remains, the leviathan can use Tentacle Torrent and Abyssal Mines but cannot move. Destroying the heart kills the leviathan.