Various Appearances. A Seak King can look like anything, from a serpentine creature to a frog, or even a bird like creature.

Sentinels of the Mermaid Princess. Sea Kings are capable of speaking to and understanding the mermaid princess, and will obey her commands.

Multiattack. The sea king makes two attacks: One with its bite and one with either its slam or tail attacks. 

Bite. Melee weapon attack: +12 to hit, reach 10ft., one target. Hit 22 (3d8 + 9) piercing damage. If the target is a large or smaller, it must succeed on a DC 18 Dexterity saving throw or be swallowed by the Sea King. A swallowed creature is blinded and restrained, it has total cover against attacks and other effects outside the Sea King, and it takes 21(6d6) acid damage at the start of each of the sea king’s turns.

If the sea king takes 30 damage or more from a creature inside it, the Sea King must succeed on a DC 21 Saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 20 feet of the Sea King.

If the sea king dies, a swallowed creature is no longer restrained by it and can escape from the corpse using 5 feet of movement, exiting prone.

Tail. Melee Weapon Attack: +12 to hit, reach 15 ft., one target. Hit: 30 (5d8 + 7) bludgeoning damage.

Slam. (Recharge 5-6) Melee Weapon Attack: +12 to hit, reach 15 ft., one target. Hit: 77 (20d6 +7) bludgeoning damage.