Aetherius, the Corruptor

Innate Spellcasting. Aetherius's spellcasting ability is Charisma (spell save DC 20, +18 to hit with spell attacks). Aetherius can innately cast the following spells, requiring no material components:

• At will: Detect Magic, Dispel Magic, Counterspell
• 1/day each: Time Stop, Mass Heal

Legendary Resistance (5/Day). If Aetherius fails a saving throw, he can choose to succeed instead.

Soul Absorption (2/day). Aetherius can use an action to absorb the essence of any of his summoned creatures within 60 feet of him. For each creature absorbed, Aetherius regains hit points equal to the creature's remaining hit points. The creature is destroyed in the process.

Regeneration. Aetherius regains 10 hit points at the start of each of his turns. If Aetherius takes radiant damage, this trait doesn’t function at the start of his next turn.

Multiattack. Aetherius makes two attacks: one with his Corruptive Claw and one with his Abyssal Bite, or one with his Void Breath (Void Breath uses both attacks).

Corruptive Claw. Melee Weapon Attack, cannot be used in True Form: +5 to hit, reach 5 ft., one target. Hit: 3d6 piercing damage. The target must succeed on a DC 16 Constitution saving throw or become corrupted. A corrupted creature takes 2d4 necrotic damage at the start of its turn and has disadvantage on saving throws. The corruption can only be cured by a Greater Restoration spell or more powerful magic.

Abyssal Bite. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 3d6 + 2 piercing damage plus 1d4 necrotic damage.

Void Breath (2 Charges, 10 second recharge period. This uses both attacks and cannot be used two turns in a row). Aetherius exhales a wave of corruptive energy in a 60-foot cone. Each creature in that area must make a DC 16 Dexterity saving throw, taking 4d8 necrotic damage on a failed save, or half as much on a successful one. 

Reality Tear (Twice per day). Aetherius rips the fabric of reality in a 60-foot line that is 5 feet wide. Each creature in that line must make a DC 16 Dexterity saving throw, each creature is pushed back 10 feet regardless of their success or failure and will take 2d12 +5 force damage and on a failed save, or half damage as much on a successful one. Aetherius can immediately teleport to any area he can see within 120 feet of him after using this attack.

Dismiss Abyssal Underling.

Opportunity Attack- Cannot be used in True Form.

Aetherius 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. Aetherius regains spent legendary actions at the start of his turn.

• Aura of Corruption 
• Summon Abyssal Underling
• Dark Pulse

Aura of Corruption. Aetherius emits a 20-foot radius aura of corruption. Creatures of his choice that start their turn in this aura must make a DC 16 Constitution saving throw or take 2d4 necrotic damage and have disadvantage on attack rolls until the start of their next turn. Creatures that succeed on the saving throw take half damage and do not suffer disadvantage.

Summon Abyssal Underling. Aetherius summons one of the following which have been corrupted by his aura, and therefore have disadvantage on all attack throws during its first turn after being summoned. Underlings take their turns at the end of the current initiative but prior to any lair actions. Aetherius can summon a shadow demon or a barlgura to engage in combat. Aetherius can summon any lessor demon (25hp), greater demon (45hp), or draconic spirit (85hp) for the sole purpose of soul absorption.

Dark Pulse (Costs 2 Actions of his next turn and reduces movement speed to 5ft for until the end of his next turn). Aetherius releases a wave of dark energy. All creatures of his choice within 60 feet must make a DC 18 Constitution saving throw, taking 4d8 necrotic damage on a failed save, or half as much on a successful one. Creatures that fail the save are also stunned until the end of their next turn.

If Aetherius is reduced to 0 hit points, he doesn’t die or fall unconscious. Instead, he sheds his physical form and reveals his godlike True Form, a small housecat. His AC increases to 19, his Dex increases to 25, he regains 250 hit points, his overall HP is increased to 900, and gains access to the following actions:

Corruptive Tentacle Melee Weapon Attack: +2 to hit, reach 20 ft., one target. Hit: 2d6 piercing damage. Aetherius releases multiple tenacles from his mouth. This attack may hit up to two creatures. 

Cosmic Ruin (Costs 2 Actions). Aetherius calls down meteors from the heavens. Each creature of his choice within a 20-foot radius must make a DC 18 Dexterity saving throw, taking 3d10 bludgeoning damage and 2d8 fire damage on a failed save, or half as much on a successful one.

Abyssal Rift (Costs 2 Actions). Aetherius tears open a rift to the Abyss. All creatures within 60 feet must make a DC 18 Constitution or Dexterity saving throw or be sucked into the rift, taking 3d8 force damage and being teleported to a random location within his lair and take 2d4 fall damage. Creatures that succeed take half damage and are not teleported.