Ancient Chuul (Leviathan Chuul)

Amphibious. The ancient chuul can breathe air and water.

Freedom of Movement. The ancient chuul 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 ancient chuul deals double damage to objects and structures.

Multiattack. The ancient chuul makes one bite or tail attack and two pincer attacks, each of which it can replace with one use of Fling. If the ancient chuul is grappling a creature, the ancient chuul can also use its tentacles once.

Bite. Melee Weapon Attack: +14 to hit, reach 5 ft., one target. Hit: 23 (3d8 + 10) piercing damage. If the target is a Large or smaller creature grappled by the ancient chuul, that creature 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 ancient chuul, and it takes 35 (10d6) acid damage at the start of each of the ancient chuul's turns.

If the ancient chuul takes 30 damage or more on a single turn from a creature inside it, it must succeed on a DC 25 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 ancient chuul. If the ancient chuul dies, a swallowed creature is no longer restrained by it and can escape from the corpse using 15 feet of movement, exiting prone.

Tentacles. One creature grappled by the chuul must succeed on a DC 17 Constitution saving throw or be poisoned for 1 minute. Until this poison ends, the target is paralyzed. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Pincer. Melee Weapon Attack: +14 to hit, reach 20 ft., one target. Hit: 20 (3d6 + 10) piercing damage, and the target is grappled (escape DC 17). Until this grapple ends, the target is restrained. The ancient chuul has two pincers, each of which can grapple one target.

Fling. One Large or smaller object held or creature grappled by the ancient chuul is thrown up to 60 feet in a random direction and knocked prone. If a thrown target strikes a solid surface, the target takes 3 (1d6) bludgeoning damage for every 10 feet it was thrown. If the target is thrown at another creature, that creature must succeed on a DC 17 Dexterity saving throw or take the same damage and be knocked prone.

Tail. Melee Weapon Attack: +14 to hit, reach 20 ft., one target. Hit: 23 (2d12 + 10) bludgeoning damage.

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

Pincer Attack or Fling. The ancient chuul makes one pincer attack or uses its Fling.

Tail Attack. The ancient chuul makes one tail attack

Sonic Boom (Costs 2 Actions). While underwater, the ancient chuul snaps its pincers shut with lightning speed, creating a sonic boom. Each creature in a 50-foot cone must succeed a DC 20 Constitution saving throw, taking 16 (3d10) thunder damage on a failed save and become stunned. A creature who succeeds the saving throw takes half as much damage, and is not stunned. The ancient chuul cannot use this attack if both pincers are currently grappling a target.