Cracken

Legendary Resistance. 3 per long rest

Amphibious. The kraken can breathe air and water.

Freedom of Movement. The kraken 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.

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

Lair. The Cracken is found in an underwater cave. The floor has many cracks and holes that reveal swirling deep water. The top of the cavern is about 150 ft up, the Cracken can summon a lightning storm in the cavern (see Lair Actions).

Multiattack. The kraken makes four tentacle attacks, each of which it can replace with one use of Fling. Or it makes 1 bite attack and 2 tentacle attacks.

Tentacle. Melee Weapon Attack: +8 to hit reach 30 ft., one target. Hit: 8 [rollable](1d10+ 5) bludgeoning damage, and the target is must make a DC 13 STR save to avoid being grappled. Until this grapple ends, the target is restrained.  A grappled creature may attempt a DC 15 DEX or STR check to escape the grapple.

Fling. One Large or smaller object held or creature grappled by the Kraken is thrown up to 60 feet in a direction and knocked prone. If a thrown target strikes a solid surface, the target takes 1d6 bludgeoning damage for every 10 feet it was thrown. If the target is thrown at another creature, that creature must succeed on a DC 16 Dexterity saving throw or take the same damage and be knocked prone.

Bite. Melee Weapon Attack: +10 to hit reach 10 ft. one target. Hit: 17 (3d8 + 5) piercing damage. If the target is a Large or smaller creature grappled by the Kraken, that creature 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 Kraken, and it takes 2d4 acid damage at the start of each of the Kraken's turns.

If the kraken takes 18 damage or more on a single turn from a creature inside it, the kraken must succeed on a DC 25 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 kraken. 

 Fury of the Sea. (Recharge 6) The Karken casts the spell Lightning Bolt or Tidal Wave originating from the Cracken or any of its tentacles. DC 15

The cracken 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 cracken regains spent legendary actions at the start of its turn.

Tentacle Fling. (costs 1 action) The cracken makes one tentacle attack or uses its Fling.

Push Back: (costs 2 actions) If the water is raised the Cracken causes a wave that pushes all creatures within 30 ft of the Cracken 60 ft away. DC 15 Strength

Lightning. (costs 2 actions) Call down a lightning strike in a 15 ft circle if the lair's water is raised. Creatures in this area must make a DC 15 DEX save or take 2d10 lightning damage. half damage on a success.

Bite. (costs 3 actions) The Cracken makes one bite attack.