Aeonus, Sky Leviathan

Amphibious: Aeonus can breathe air and water.

Aerial Titan: Aeonus treats the air as if it was water. It can maneuver in the air as well as it can underwater and is always treated as flying, even when incapacitated.

Weathermaster: Aeonus can innately control the weather in a ten mile radius. It usually chooses gale storms 

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

Magic Weapons: Aeonus attacks are magical.

Siege Monster: Aeonus  deals double damage to objects and structures.

Spell-Resistant: Aeonus has advantage on saving throws against spells, and any creature that makes a spell attack against Aeonus has disadvantage on the attack roll.

Multiattack. Aeonus makes three attacks each turn and may choose combination of one Bite, one tail, one Lightning Bolt, and or Tidal Wave (if available) 

Tail: Melee Weapon Attack: +18 to hit, reach 20 ft., one target. Hit: 23 (3d8 + 10) bludgeoning damage, and if the target is a creature, it is knocked prone.

Lightning Bolt. Ranged Weapon Attack: +18 to hit, reach 20 ft., one creature. Hit: 48 (12d6) lightning damage, 

Bite. Melee Weapon Attack: +18 to hit, reach 5 ft., one target. Hit: 29 (3d12 + 10) piercing damage. If the target is a Large or smaller creature grappled by Aeonus, 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 Aeonus, and it takes 42 (12d6) acid damage at the start of each of Aeonus’ turns. If Aeonus takes 50 damage or more on a single turn from a creature inside it, Aeonus must succeed on a DC 20 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of Aeonus. If Aeonus dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 15 feet of movement, exiting prone.

Tidal Wave: (Recharge 5–6)  Aeonus conjure up a wave of water or air that crashes down on an area within range. The area can be up to 60 feet long, up to 20 feet wide, and up to 20 feet tall. Each creature in that area must make a Dexterity saving throw. On a failed save, a creature takes 48 (12d6) damage and is knocked prone. On a successful save, a creature takes half as much damage and isn’t knocked prone. The water then spreads out across the ground in all directions, extinguishing unprotected flames in its area and within 30 feet of it, and then it vanishes. A tidal wave of air does half the damage of a tidal wave of water. (6d6) 

Heart of the Storm  (Recharge 5–6)  As a bonus action  Aeonus conjures a churning storm cloud centered on itself. Lightning flashes in the area, thunder booms, and strong winds roar. Each creature under the cloud (no more than 5,000 feet beneath the cloud) when it appears must make a Constitution saving throw. On a failed save, a creature takes 2d6 thunder damage and becomes deafened for 5 minutes. In the following rounds these things happen as long as Aeonus spends a bonus action concentrating on the effect.

• Round 1: Sleeting rain falls from the cloud. Each creature and object under the cloud takes 6d6 cold damage.
• Round 2: Aeonus call ten bolts of lightning from the cloud to strike ten creatures or objects of its choice beneath the cloud. A given creature or object can't be struck by more than one bolt. A struck creature must make a Dexterity saving throw. The creature takes 12d6 lightning damage on a failed save, or half as much damage on a successful one.
• Round 3: Hailstones rain down from the cloud. Each creature under the cloud takes 4d6 bludgeoning damage.
• Round 4-5: Gusts and freezing rain assail the area under the cloud. The area becomes difficult terrain and is heavily obscured. Each creature there takes 2d6 cold damage. Ranged weapon attacks in the area are impossible. The wind and rain count as a severe distraction for the purposes of maintaining concentration on spells. Finally, gusts of strong wind (ranging from 20 to 50 miles per hour) automatically disperse fog, mists, and similar phenomena in the area, whether mundane or magical.

Aeonus 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. Aeonus regains spent legendary actions at the start of its turn.

Move. Aeonus moves up to half its speed.

Tail. Aeonus makes one tail attack.

Heart of the storm: Aeonus can initiate or sustain it's Heart of the Storm ability.

Bite (Costs 3 Actions). Aeonus makes one bite attack.