The Leviathan

Traits:
Amphibious. The leviathan can breathe air and water.

Freedom of Movement. The leviathan ignores difficult terrain, and magical effects can't reduce its speed or cause it to be restrained. It can spend 5 feet of movement to escape from nonmagical restraints or being grappled.

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 (included in Tidal Wave).

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.

Actions:
Multiattack. The leviathan makes two attacks: one with its slam and one with its tail.

Slam. Melee Weapon Attack: +16 to hit, reach 20 ft., one target. Hit: 15 (1d10 + 10) bludgeoning damage plus 5 (1d10) acid damage.

Tail. Melee Weapon Attack: +16 to hit, reach 20 ft., one target. Hit: 16 (1d12 + 10) bludgeoning damage plus 6 (1d12) acid damage.

Tidal Wave - Recharge . While submerged, the leviathan magically creates a wall of water centered on itself. The wall is up 250 feet long, up to 250 feet high, and up to 50 feet thick. When the wall appears, all other creatures within its area must each make a DC: 24 Strength saving throw. A creature takes 33 (6d10) bludgeoning damage on failed save, or half as much damage on a successful one.

Legendary Actions:

Multiattack. : The leviathan makes two attacks: one with its slam and one with its tail.

Slam. : Melee Weapon Attack: +16 to hit, reach 20 ft., one target. Hit: 15 (1d10 + 10) bludgeoning damage plus 5 (1d10) acid damage.

The leviathan is an elder elemental of water, capable of sweeping life from entire coastlines when called into the world. Elder elementals are elementals who’ve swelled to extreme size and power by feeding on their fellows and beings from beyond. In the material world, their presence can cause apocalyptic disasters.