Void Leviathan

Siege Monster. Damage doubled against structures.

Void Beast. Suffers no penalties and cannot have its movement speed reduced while in the void or ethereal plane.

Eldritch Nature. The Leviathan does not need air, sleep, or water to survive.

Swift Hunter. When the Leviathan deals damage to a creature on its turn, it can no longer be targeted with oppurtunity attacks by that creature until the start of that creature's next turn.

Home Turf. When in the void or ethereal plane, the Leviathan gets a +2 bonus to its AC and gets one additional legendary action per round.

Multiattack. The Leviathan makes four attacks- three with its raking claws and one more action below of its choice.

Raking Claws. Melee Weapon Attack: +18 to hit, range 25 ft., 1 target. Hit: 23 (2d12 + 10) slashing damage. If it is in the void or ethereal plane, this attack does an additional 16 (3d10) necrotic damage. This damage ignores resistance to necrotic damage, and treats immunity as resistance. When this attack hits a creature, the Leviathan may target any number of creatures within 5 feet of the Leviathan and force them to make a DC 22 Dexterity saving throw. On a failure, the creature(s) will take slashing damage equal to half the overall damage dealt to the initial target.

Abyss's Maw. Melee Weapon Attack, +18 to hit, up to three large or smaller targets within 10 feet of each other, range 5 ft. On a hit, each creature hit is swallowed by the Leviathan, dealing 44 (8d10) slashing damage as they fall down its toothy maw. At the start of each of their subsequent turns, each creature swallowed by the Leviathan takes 28 (8d6) force damage as the Void flames within dissolve the creature. A creature reduced to 0 hp in this way will be eaten by the flames, instantly dying and having its soul trapped in the Leviathan. When this happens, the Leviathan regains hit points equal to the creature's maximum hit points. If this heals the Leviathan to full health, the remaining hit points instead convert to temporary hit points. A creature inside the Leviathan can attack it from the inside, and upon dealing 50 damage, can create a hole large enough to escape, landing prone on the closest space beneath the creature.

Bite. Melee Weapon Attack. 1 target, range 5 ft, +18 to hit. On a hit, this attack does 49 (6d12 + 10) piercing damage, plus 18 (4d8) necrotic damage. If the Leviathan is in the Void, double the necrotic damage dealt by this attack while applying. This necrotic damage follows the same rules detailed under the Raking Claws action.

Chaos Burst (Recharge 6). The Leviathan unleashes the chaotic energy that makes up its spirit, sending a bright beam of crackling energy in a line 50 feet wide and 500 feet long. Any afflicted creature must make a DC 26 Dexterity saving throw or take 72 (16d8) force damage plus 27 (5d10) necrotic damage. This necrotic damage is doubled while the Leviathan is in the Void, and ignores resistances as detailed in the Raking Claws action.

Warp. The Leviathan travels to an equivalent space in the Void, Material Plane, or Ethereal Plane.

Graze. When it moves within 5 feet of a creature on its turn, the Leviathan may take this reaction to make a Raking Claws attack.

The Void Leviathan has 3 legendary actions (4 while in the void). These legendary actions can be taken at the end of another creature's turn.

Chaos Spittle (Costs 2 Actions). The Leviathan makes a Ranged Attack, shooting a bolt of chaotic energy at a creature within 120 feet of it (+16 to hit). On a hit, this attack does 26 (4d12) force damage and reduces a creature's movement speed by half. This effect cannot stack. 

Rake. (Costs 1 Action). The Leviathan makes a Raking Claws attack.

Slip. (Costs 1 Action). The Leviathan moves up to half its flying speed without provoking oppurtunity attacks.