Cetus, Lord of the Deep

Amphibious. Cetus can breathe air and water.

Freedom of Movement. Cetus 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 Cetus fails a saving throw, it can choose to succeed instead.

Limited Magic Immunity. Cetus is immune to spells of 4th level or lower unless it wishes to be affected. It has advantage on saving throws against all other spells and other magical effects.

Magic Weapons. Cetus's weapon attacks count as magical.

Regeneration. If Cetus is in contact with salt water, then Cetus regains 20 hit points at the start of its turn unless it begins its turn with 0 hit points.

Shapeshifter. Cetus can use its turn to polymorph into a large humanoid, or back into its true form. In humanoid form, Cetus cannot use legendary actions. Other than that and its size, its statistics are the same in each form. Any equipment it is wearing or carrying is also transformed. It reverts to its true form if it dies.

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

Tide Render. The Tide Render is a magical +1 legendary longsword. On a successful hit, this weapon deals an extra 2d6 lightning damage, and makes its wielder immune to cold damage while it is unsheathed. The wielder can cast maddening darkness once with this weapon (saving throw DC 20) . The wielder can only cast this once per day. The weapon recharges at dawn. (Included in attack action) (Requires attunement)

Multiattack. Cetus can use its Command Sea Creatures ability. It then makes one bite attack and three tentacle or Tide Render attacks, each of which it can replace with one use of Fling.

Bite. Melee Weapon Attack: +17 to hit, reach 5 ft., one target. Hit: 21 (2d10 + 10) in humanoid form, or 26 (3d10 + 10) piercing damage in its true form. If Cetus is in its true form, and 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 42 (12d6) acid damage at the start of each of the kraken's turns.

If the kraken takes 50 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. If the kraken dies, a swallowed creature is no longer restrained by it and can escape from the corpse using 15 feet of movement, exiting prone.

Tentacle. Melee Weapon Attack: +17 to hit, reach 30 ft., one target. Hit: 23 (3d8 + 10) bludgeoning damage, and the target is grappled (escape DC 18). Until this grapple ends, the target is restrained. The kraken has ten tentacles, each of which can grapple one target.

Tide Render (Humanoid Form Only). Melee Weapon Attack: +18 to hit, reach 10 ft., one target.  Hit: 19 (2d8 + 10) slashing damage plus 7 (2d6) lightning damage, or 22 (2d10 +11) slashing damage plus 7 (2d6) lightning damage with both hands.

Fling (True Form Only). One Large or smaller object held or creature grappled by the kraken is thrown up to 60 feet in a random direction and knocked prone. If a thrown target strikes a solid surface, the target takes 3 (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 18 Dexterity saving throw or take the same damage and be knocked prone.

Command Sea Creatures. Cetus commands aquatic creatures to do its bidding. This affects all aberrations, beasts, and oozes within 240 feet that are able to breathe water (a DC 22 Wisdom saving throw negates). This functions like mass suggestion, but can affect mindless creatures. Cetus can suggest obviously harmful or suicidal acts (though non-mindless creatures have advantage and a +5 bonus on their saving throws against these suggestions). The commanded course of activity can have a duration of up to 1 hour. If Cetus issues a new command to a creature, the previous command is discarded. A creature that successfully resists this effect is immune to further commands from Cetus for 24 hours.

Lightning Storm. The kraken magically creates four bolts of lightning, each of which can strike a target the kraken can see within 120 feet of it. A target must make a DC 23 Dexterity saving throw, taking 22 (4d10) lightning damage on a failed save, or half as much damage on a successful one.

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

Tentacle Attack or Fling. The kraken makes one tentacle attack or uses its Fling.

Lightning Storm (Costs 3 Actions). The kraken uses Lightning Storm.

Ink Cloud (Costs 3 Actions). While underwater, the kraken expels an ink cloud in a 60-foot radius. The cloud spreads around corners, and that area is heavily obscured to creatures other than the kraken. Each creature other than the kraken that ends its turn there must succeed on a DC 23 Constitution saving throw, taking 16 (3d10) poison damage on a failed save, or half as much damage on a successful one. A strong current disperses the cloud, which otherwise disappears at the end of the kraken's next turn.