Atlarum Leviathan

Amphibious. The leviathan can breathe air and water.

Atlarum Steel. The leviathan has a damage threshold of 15; it has immunity to all damage unless it takes an 15 or more damage, in which case it takes damage as normal.

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 colossus has advantage on saving throws against spells and other magical effects.

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

Slow Healing. When the leviathan takes a long rest, it does not recover hit points unless it spends hit die as if taking a short rest. When it completes its long rest, it recovers 1d4 hit die, instead of the usual amount. 

Multiattack. The leviathan makes 3 claw attacks.

Action Melee Attack. Melee Weapon Attack: +16 to hit, reach 20 ft., 1 target. Hit: 28 (4d8 + 10) slashing damage. 

Lightning Beam (Recharge 5-6). Ranged Spell Attack: +15 to hit, range 120/600 ft., 1 target. Hit: 99(18d10) lightning damage.

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.

Claw Attack. The leviathan makes one claw attack.

Detect. The leviathan makes a Wisdom (perception) check.

Tail Attack (Costs 2 Actions).  The leviathan beats its tail. Each creature within 30 feet of the leviathan mist succeed on a DC 21 Dexterity saving throw or take 17 (2d6 + 10) bludgeoning damage and be knocked prone. The leviathan can then swim up to half its swim speed.