Freedom of Movement. The leviathan 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.

Legendary Resistance (3/Day). If the leviathan fails a saving throw, it can choose to succeed instead.

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

Water Breathing. The leviathan can breathe only underwater.

Multiattack. The leviathan can use its Frightful Presence. It then makes five attacks, one with its bite and four with its tentacles, each of which it can replace with one use of Crush.

Bite. Melee Weapon Attack: +19 to hit, reach 5 ft., one target. Hit: 36 (4d12+10) piercing damage. If the target is a creature, it is grappled (escape DC 20). Until the grapple ends, the target is considered restrained, and the leviathan can't bite another target. If the target is a creature grappled by the leviathan, 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 leviathan, and it takes 49 (14d6) acid damage at the start of each of the leviathan's turns.

If the leviathan takes 60 damage or more on a single turn from a creature inside it, the leviathan 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 leviathan. If the leviathan dies, a swallowed creature is no longer restrained by it and can escape from the corpse using 30 feet of movement, exiting prone.

Tentacle. Melee Weapon Attack: +19 to hit, reach 30 ft., one target. Hit: 24 (4d6+10) bludgeoning damage, and the target is grappled (escape DC 20). Until this grapple ends, the target is restrained. The leviathan has six tentacles, each of which can grapple one target.

Crush. One object held or creature grappled by one of the leviathan's tentacles is squeezed to the point of bursting. The creature make make a DC 25 Constitution saving throw, taking 28 (4d8+10) bludgeoning damage on a failed save, or half as much on a successful one.

Frightful Presence. Each creature of the leviathan's choice within 120 feet of it and aware of it must succeed on a DC 22 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, with disadvantage if the leviathan is within line of sight, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the leviathan's Frightful Presence for the next 24 hours.

Energy Storm. The leviathan magically creates six bolts of pure energy, each of which can strike a target the leviathan can see within 120 feet of it. A target must make a DC 25 Dexterity saving throw, taking 22 (4d10) radiant damage on a failed save, or half as much damage on a successful one.

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

Tentacle Attack or Crush. The leviathan makes one tentacle attack or uses its Crush.

Consume (Costs 2 Actions). The leviathan makes one bite attack against a target it currently has grappled.

Energy Storm (Costs 2 Actions). The leviathan uses Energy Storm.

Energy Pulse (Costs 3 Actions). The leviathan expels a powerful blast of energy in a 90-foot radius. This spreads around corners. Each creature other than the leviathan must succeed on a DC 25 Constitution saving throw, taking 22 (4d10) radiant damage on a failed save, or half as much damage on a successful one.