Atlantean Leviathan

Empowered Attacks. The leviathan's attacks are treated as magical and adamantine for the purpose of bypassing resistance and immunity to nonmagical weapons.

Immutable Form. The leviathan is immune to any spell or effect that would alter its form.

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

Magic Resistance. The leviathan has advantage on saving throws against spells and other magical effects.

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

Multiattack. The leviathan makes four attacks: two with its greater claws and two with its lesser claws.

Greater Claw. Melee Weapon Attack: +19 to hit, reach 500 ft., one target. Hit: 115 (10d20 + 10) slashing damage, and the target is grappled (escape DC 27). The leviathan has two greater claws, each of which can grapple only one target. This attack has disadvantage against targets Huge and smaller.

Lesser Claw. Melee Weapon Attack: +19 to hit, reach 100 ft., one target. Hit: 75 (10d12 + 10) slashing damage, and the target is grappled (escape DC 27). The leviathan has two lesser claws, each of which can grapple only one target.

Beam Weapon (Recharge 5–6). The leviathan fires a destructive ray in a 5,000 foot line that is 15 feet wide. Each creature in that line must make a DC 27 Dexterity saving throw, taking 78 (12d12) 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.

Attack. The leviathan makes one melee attack.

Move. The leviathan moves up to its speed.

Sweep (Costs 3 Actions). The leviathan makes a greater claw attack against each creature within 500 feet of it.