Leviathan, Lord of Envy

Siege Monster. Leviathan deals double damage to Objects and Structures.
Echolocation. Releasing a booming guttural roar Leviathan can sense anything within 1 mile of it, so long as it is in water. 
Magic Resistance. The Leviathan has advantage on saving throws against spells and other magical effects.
Heavy Current. Leviathan's body is so massive, the waves it can cause just from moving can be destructive enough to knock ships over or out of position, but not enough to destroy one.
Hellmaw. Any creature(s) swallowed by Leviathan are transported to the First Circle of Hell

Bite. Melee Weapon Attack: +19 to hit, reach 20 ft., one target. Hit: 36 (4d12 + 10) Piercing damage. If the target is a Huge or Smaller creature, that creature is grappled While grappled it takes 32 (10d6) piercing damage at the start of each of Leviathan's turns. If Leviathan takes 50 damage or more on a single turn from a creature inside it, Leviathan must succeed on a DC 20 Constitution saving throw at the end of that turn or release the target, which falls Prone.

Hurl. One Huge or smaller held or creature grapples by Leviathan is thrown to the nearest vertical surface. If the target is thrown at another creature, that creature must make a DC 17 Dexterity saving throw or take the same 13(1d6 +10) for every 10 feet traveled

Slam. Melee Weapon Attack: +17 to hit, range 15 ft., four targets. Hit: 26 (3d10 + 9) bludgeoning damage. The target(s) must succeed on a DC 20 Dexterity saving throw, or be stunned for 1 minute. If they succeed, they will not be stunned and take only half the damage from the attack.

Ship-Breaker.  Leviathan chooses one ship within 120 ft. and throws his bulk through it. The target must make a DC 20 Strength saving throw, completely destroying the ship on a failed save, or immunity on a successful one.

Frightful Presence. Each creature of Leviathan's choice that is within 120 feet of the dragon and aware of it must succeed on a DC 18 Wisdom saving throw or becomes Frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, 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 Leviathan's Frightful Presence for the next 12 hours.

Swift Swimming. Leviathan goes underwater. For this turn, attacks have a -2 modifier to hit. Movement: 500 ft.

Tidal Wave. With a swift movement of its body Leviathan can make up to a 30 feet long, 10 feet wide, and 10 feet tall Wave. Each creature in the area must make a DC 17 Dexterity save. On a failure, it takes 4d8 bludgeoning damage and is knocked prone. On a success, it takes half damage and isn't prone. The water spreads across the ground in all directions, extinguishing unprotected flames in its area. If a Tsunami is active this action may not be taken.

Envy of the Deep (3/day). Leviathan swims 1000 feet downwards in a circular fashion, creating a whirlpool with a 150 feet radius, affecting every creature and loose object or structure within it. Every target must then succeed a DC 23 Strength saving throw or be restrained until the end of Leviathans turn, also, if the saving throw was failed, the target will be dragged downwards by the whirlpool. At the end of the Leviathans turn, all targets that failed the saving throw are then within a 10 ft. reach of Leviathan. Large and smaller creatures or objects are immediately dragged down to the depths. This action immediately ends any ongoing Tsunami's within the radius of the Whirlpool.

Tsunami (Recharge 19-20). Using all of its movement and action Levithan creates a wall of water in front of it. Leviathan can make the wall up to 300 feet long, 300 feet high, and 50 feet thick. The wall lasts for the duration of 6 rounds. When the wall appears, each creature within its area must make a DC 20 Strength saving throw. On a failed save, a creature takes 6d10 bludgeoning damage, or half as much damage on a successful save. 
At the start of Leviathan's turns after the wall appears, the wall, along with any creatures in it, moves 50 feet away from you. Any Huge or smaller creature inside the wall or whose space the wall enters when it moves must succeed on a DC 20 Strength saving throw or take 5d10 bludgeoning damage. A creature can take this damage only once per round. At the end of the turn, the wall's height is reduced by 50 feet, and the damage creatures take from the spell on subsequent rounds is reduced by 1d10. When the wall reaches 0 feet in height, the spell ends. If Tidal wave was cast within the last 10 rounds, the max height of the wave is 150ft.

Leviathan can take 2 legendary action, choosing from the options bellow. It can use these legendary action more than once if the cost allows but only at the end of another creatures turn. Leviathan can regain spent legendary action at the end of its turn

x1 : Hurl, Swift Swimming, Frightful Presence

x2 Envy of the Deep, Tidal Wave, Slam