Cetus

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.

Amphibious Cetus can breathe both air and water.

Siege Monster All attacks deal double damage to objects and structures.

Vulnerable skin Cetus is covered in armor plating, except in 3 segments, which expose the creature's flesh. Cetus's skin has an armor class of 15, and attacks on these parts deal double damage. Cetus's skin is not immune to non-adamantine weapons. After taking 50 or more damage, this segment shatters, and attacking this segment will have the normal AC/resistances.

Legendary Resistance (3/day) If Cetus fails a saving throw, it can choose to succeed instead.

Brute attacks. For the purpose of overcoming resistance, Cetus's attacks count as magical.

Multiattack. Cetus has 3 attack charges per turn. Cetus can expend one charge to make either a Bite, Tail, or Mine attack. Cetus can't attack a creature with its Bite and Tail in the same turn. Cetus can use two charges to make a Vile Vomit attack. 

Bite. Melee Weapon Attack: +12 to hit, reach 10 ft., one target. Hit: 19 (2d8+10) Piercing damage. If the target is a Large or smaller creature, that creature is swallowed. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside Cetus, and it takes 35 (10d6) acid damage at the start of each of the Cetus's turns.

If Cetus takes 40 damage or more on a single turn from a creature inside it, Cetus must succeed on a DC 23 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of Cetus. If Cetus dies, a swallowed creature is no longer restrained by it and can escape from the corpse using 15 feet of movement, exiting prone.

Tail. Melee Weapon Attack: +12 to hit, reach 20 ft., one target. Hit: 20 (3d6+10) Bludgeoning damage, and the creature must succeed on a DC 19 Strength saving throw or be pushed 10 feet away from Cetus.

Mines. For every 10 feet that Cetus travels, it releases a mine. Mines explode after 2 rounds, when a creature other than Cetus moves into its space, or are hit with any damage. Mines are a 2.5 ft radius sphere of metal and have an AC of 10. Upon explosion, each creature within 5 feet of the mine must make a DC 15 Dexterity saving throw, taking 7 (2d6) Piercing damage and 7 (2d6) Bludgeoning Damage on a failed save, or half as much on a successful one.

Vile Vomit (Recharge 5-6) Cetus vomits hundreds of tiny, biting, sea creatures in a 90-foot cone. Each creature in that area must make a DC 19 Constitution saving throw, taking 70 (20d6) piercing damage on a failed save, or half as much damage on a successful one. Upon use of this ability, Cetus also releases any creature it had swallowed.

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. Cetus regains spent legendary actions at the start of its turn. 

Rip Current (Costs 2 Actions). Cetus uses its movement speed to swim around in a 12.5 ft. radius circle. Creatures inside the circle must make a DC 19 Strength saving throw, becoming restrained and taking 22 (4d8+4) Bludgeoning damage on a failed save, or half as much on a successful save, and not becoming restrained. Outside the circle, creatures 10 ft away from the circle must make a DC 19 Str saving throw, being knocked prone, taking 18 (4d6+4) bludgeoning damage on a failed save, or half as much damage and not being knocked prone on a successful save.

Bite. Cetus uses its Bite attack

Detect. Cetus makes a Perception check.

Refill. Cetus regains the use of its Vile Vomit attack.