Cetus

Amphibious. The cetus can breathe both air and water.

Innate Spellcasting. The cetus' innate spellcasting ability is Wisdom (Spell save DC 19, +11 to hit with spell attacks). It can innately cast the following spells, requiring no material or verbal components:

At Will: Create or Destroy Water, Control Water, Fog Cloud
3/Day Each: Ice Storm, Water Breathing, Water Walk
1/Day: Storm of Vengeance

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

Ocean's Fury (Recharges after a Short or Long Rest). When the cetus would be reduced to 0 hit points, its current hit points instead reset to 175, it immediately recharges and uses its Tidal Charge, and it regains one expended use of its Legendary Resistance. Additionally, the cetus can now use the options in the "Mythic Actions" section for 1 hour. Award a party an additional 22,000 XP (44,000 XP total) for defeating the cetus after its Ocean's Fury trait activates.

Blessing of Poseidon. The cetus has advantage on saving throws against spells and other magical effects. Additionally, the cetus regenerates 20 hit points at the start of its turn, provided it is submerged in water and has at least 1 hit point.

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

Sonar Sense. While fully submerged in water, the cetus has Blindsight out to 120 ft. (included above). The cetus can't use this sense while it has the Deafened condition.

Terrifying Presence. Any creature that starts its turn within 60 ft. of the cetus and can see it must succeed on a DC 18 Wisdom saving throw or have the Frightened condition for 1 minute. A frightened creature can repeat its saving throw at the end of each of its turns (with disadvantage if the cetus is within line of sight), ending the effect on itself on a success.

Multiattack. The cetus makes two attacks: one with its Bite and one with its Tail Swipe. It can choose to replace its Bite with Devour.

Bite. Melee Weapon Attack: +15 to hit, reach 15 ft., one target. Hit: 31 (4d10+9) piercing damage. If the target is a Large or smaller creature, it has the Grappled condition (escape DC 21). Until this grapple ends, the target has the Restrained condition, and the cetus can't use its Bite on another target.

Tail Swipe. Melee Weapon Attack: +15 to hit, reach 20 ft., one target. Hit: 27 (4d8+9) bludgeoning damage. If the target is a Huge or smaller creature, it must succeed on a DC 21 Strength saving throw or have the Prone condition.

Devour. Melee Weapon Attack: +15 to hit, reach 5 ft., one creature grappled by the cetus. Hit: the target is swallowed and the grapple ends. A swallowed creature has the Blinded and Restrained conditions, it has total cover against attacks and other effects outside the cetus, and it takes 35 (10d6) acid damage at the start of each of the cetus' turns. If the cetus takes 40 damage or more on a single turn from a creature inside it, the cetus must succeed on a Constitution saving throw with a DC equal to half the damage taken (rounded up) at the end of that turn or regurgitate the swallowed creature, which falls Prone in a space within 10 ft. of the cetus. If the cetus dies, a swallowed creature is no longer restrained by it, and can escape from the corpse by using 30 ft. of movement, exiting Prone.

Tidal Charge (Recharge 4-6). The cetus charges in a straight line up to 80 ft. while underwater. Each creature in its path must make a DC 21 Dexterity saving throw, taking 35 (10d6) bludgeoning damage and gaining the Prone condition. On a success, a creature takes half as much damage only.

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

Move. The cetus moves up to half its movement speed without provoking opportunity attacks.

Tail Swipe (Costs 2 Actions). The cetus makes one Tail Swipe attack.

Devour (Costs 3 Actions). The cetus uses Devour.

If the cetus' Coean's Fury trait has activated in the last hour, it can use the options below as legendary actions.

Tidal Charge (Costs 2 Actions). The cetus uses Tidal Charge (If available).

Whirlpool Maelstrom (Costs 3 Actions). The cetus creates a whirlpool in a 60 ft. radius centered on itself. Creatures in that area must make a DC 21 Strength saving throw, taking 42 (12d6) bludgeoning damage on a failure, or half as much damage on a success. A creature that fails its save is also pulled toward the cetus, and has the Restrained condition until the end of its next turn.