Chaosfell Leviathan

Amphibious. The leviathan can breathe air and water.

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

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

Lightning Supremacy. Lightning damage dealt by the leviathan bypasses lightning resistance and deals half damage to creatures that are immune to lightning damage.

Lightning Aura. Lightning sparks off of the leviathan into the surrounding area. Any creature that ends its turn within 5 feet of the leviathan takes 7 (2d6) lightning damage. If the leviathan is submerged in water, the range of its lightning aura is increased to 30 feet.

Multiattack. The Leviathan can do two attacks either two rend attacks or a Thunderous body slam attack.

Rend. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 18 (3d8 + 5) piercing damage + 7 (2d6) lightning damage.

Thunderous Body slam. Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 21 (3d10 + 5) bludgeoning damage plus 10 (3d6) lightning damage. If the leviathan moved at least 20 feet straight toward the target immediately before the hit, the target takes an extra 16 (3d10) bludgeoning damage. If the target is a creature, it must succeed on a DC 19 Strength saving throw or be pushed up to 10 feet away and knocked prone.

Thunderbolt. Ranged Weapon Attack: +11 to hit, range 80/320 ft., one target. Hit: 7 (2d6) lightning damage. Hit or miss, the thunderbolt then explodes. The target and each creature within 10 feet of it must succeed on a DC 20 Dexterity saving throw or take 21 (6d6) lightning damage and be afflicted with the Stunned condition on a failed save, or half as much damage and isn't afflicted with Stunned condition on a successful one. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Lightning Breath (Recharge 5-6). The Leviathan exhales lightning in a 90-foot line that is 10 feet wide. Each creature in that line must make a DC19 Dexterity saving throw. On a failed save, the creature takes 60 (11d10) lightning damage and is afflicted with the Paralysis condition. On a successful save, the creature takes half as much damage and isn't afflicted with paralysis condition. A creature can make a DC 18 Constitution saving throw at the end of each of its turns, ending the effect on a success.

Electric discharge. The Leviathan releases electricity in a 30-foot radius around it. Each creature in that area must make a DC 19 Dexterity saving throw, taking 22 (4d10) lightning damage and be afflicted with Paralysis condition for 1 minute on a failed save, or half as much damage and is not afflicted with Paralysis 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 two Thunderbolt attacks, or it moves up to 10 feet and then makes one Rend attack.

Lightning Charge (Costs 2 Actions). The leviathan moves up to its speed in a straight line. During this move, it can move through the spaces of other creatures without provoking opportunity attacks. Each creature the leviathan moves through must succeed on a DC 19 Dexterity saving throw or take 18 (3d8 + 5) bludgeoning damage plus 10 (3d6) lightning damage.

Lightning ball (Cost 2 Actions). The leviathan exhales a ball of lightning to a location with 60 feet of it. It then bounces two additional times, landing in a space 20 feet away from the previous location. Each creature within a 10-foot radius of any of those locations must make a DC 20 Dexterity saving throw, taking 60 (11d10) lightning damage on a failed save or half as much damage on a successful one.