Cave Dwelling Leviathan

The leviathan has made its home in the dark, damp depths of an underground cave that is connected to the ocean through underground passages. 

Creature of land & sea. Originally born in the ocean, the leviathan emerged on to land and evolved. The leviathan can breath on land and underwater. 

Siege Monster. The leviathan deals double damage to objects and structures. 

Multiattack. The leviathan can make three attacks as an action. Two with its claws and one with its bite attack.

Lair Actions

The leviathan can retreat into the depths of the water and prepare for its next attack.

Breach attack. The leviathan bursts out of the water to attack a target within 10 feet of the water. It makes a single claw attack that deals double damage on a hit due to the momentum of the leviathan emerging from the water. On a hit the target is knocked prone and must make a Constitution saving throw (DC 18). On a fail the target is stunned for its next turn.

Fire Breath (Recharge 5-6). The leviathan breathes a 30 foot cone of intensely hot blue fire, even underwater. All creatures in the area must make a DC 18 Dexterity saving throw, taking 8d6 fire damage on a failed save, or half as much damage on a successful one.

Claw. Melee weapon attack: +12 to hit, reach 15ft., one target. Hit: 3d6+9 slashing damage. 

Bite. Melee weapon attack: +12 to hit, reach 10 ft., one target. Hit: 3d12+9 piercing damage. If the bite attack is successful the target must make a Dexterity save of 18 or become grappled. The target may attempt to break free from the leviathan's grasp on their next turn (DC 18 strength). Until this grapple ends, the leviathan can't bite another target. 

Swallow. (bonus action) The leviathan can attempt to swallow any large or smaller target that is grappled by its bite. The grappled target is thrown into the air as the leviathan opens its mouth to swallow it. The target may attempt a Dexterity save (DC 18) to try and avoid being swallowed. On a successful save, the target avoids being swallowed, falls to the ground and is no longer grappled. On a failed save, the target is swallowed until a DC 20 strength check allows the target to climb back out the leviathan's mouth. While swallowed, the target is blinded and restrained, it has total cover against attacks and other effects from the outside and take 8d6 acid damage at the start of each turn. If the leviathan takes 40 or more damage on a single turn from the swallowed target, it must succeed on a DC 20 Constitution saving throw or regurgitate all swallowed targets, which fall prone in a space within 10 feet of the leviathan. If the leviathan dies, a swallowed target can use half its movement to crawl out, exiting prone. 

Attack of opportunity. The leviathan can make one claw attack on a target that exists its range. 

Hypnotic trance. The leviathan uses its glowing antenna to control its enemies. As an action, the leviathan can attempt to hypnotise a single target. The target must make a wisdom saving throw (DC 18). On a failed save the target becomes hypnotised and is controlled by the leviathan. At the beginning of each of the targets turns, the target must attempt the save again to try and break free of the hypnosis. Whilst being controlled, the target must do as the leviathan telepathically commands. The hypnosis ends if the target succeeds on the save or the leviathan is killed.