Cetaceaus Magnus

Leviathan. The cetaceaus magnus has advantage on all STR checks (including Athletics and Grapple) made either against a static DC or against a creature smaller than itself. Against medium creatures, it gains a +2 to any STR checks, and against a small or tiny creature, it gains a +4.

Amphibious. The cetaceaus magnus can holds its breath for up to one hour. When in the water, its DEX is 16.

Multi-Attack. One strike and one bite per turn. In lieu of attacking, it can cast its known spells.

Melee Attack. Strike: +8 to hit, reach 10 ft., 1 target. Hit: 16 (3d6 + 6) bludgeoning damage.

Melee Attack. Bite: +8 to hit, reach 10 ft., 1 target. Hit: 10 (1d6 + 6) piercing damage + 1d4 acid damage. On a successful hit, the target is automatically grappled. If the target remains grappled for three consecutive rounds, it is swallowed. A swallowed creature is stunned for one round and takes 8 (3d4) acid damage each turn until it dies or the cetaceaus magnus is killed. When it is no longer stunned it can make one attack action against the cetaceaus magnus per round at disadvantage, but can take no other actions.

Command of the Waves. Cast Wall of Water and Watery Sphere once each per long rest.