Colossal Mosasaurus

Hold Breath: The mosasaur can hold it's breath for 5 hours.

Freedom of Movement: The mosasaur 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 mosasaur deals double damage to objects and structures.

Multiattack. The mosasaur makes two attacks: one with its bite and two with its claws. 

Bite. Melee Weapon Attack: +13 to hit, reach 15 ft., one target. Hit: 30 (4d10 + 7) piercing damage.

Tail. Melee Weapon Attack: +13 to hit, reach 15 ft., one target. Hit: 26 (3d12 + 7) bludgeoning damage. If the target is a creature, it must succeed on a DC 20 Strength saving throw or be pushed up to 10 feet away from the mosasaur and knocked prone.

Radioactive Bite. Melee Weapon Attack: +13 to hit, reach 15 ft., one target. Hit: 30 (4d10 + 7) piercing damage plus 12 (2d8) poison damage.

Swallow. The mosasaur makes one bite attack against a Large or smaller creature it is grappling. If the attack hits, the target takes the bite's damage, the target 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 mosasaur, and it takes 36 (8d6) acid damage at the start of each of the mosasaurs turns.

If the mosasaur takes 30 damage or more on a single turn from a creature inside it, the mosasaur must succeed on a DC 20 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 mosasaur. If the mosasaur dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 30 feet of movement, exiting prone.