Legendary Resistance (6/Day). Whenever Aabaloth fails a Saving Throw, he can choose to succeed instead.

Magic Resistance. Aabaloth has Advantage on Saving Throws against spells and other magical effects. 

Special Equipment. Aabaloth wields two daggers: Afterthought and Second Thought. These magical weapons count as +3 Shortswords when used by any creature except of Aabaloth. 

Antilife Aura. All creatures within 60 ft. of Aabaloth cannot regain Hit Points and receive Vulnerability to Necrotic damage until they leave the aura. 

Dread Presence. When Aabaloth takes his turn, all creatures within 60 ft. must succeed a DC 17 Wisdom Saving Throw or take 20 Psychic damage and become Frightened for 3 turn. Aabaloth lands guaranteed Critical Hits against creatures that are Frightened in this way.

Ghastly Form. All creatures within 30 ft. of Aabaloth take 10 Necrotic damage per turn and suffer Disadvantage on Death Saving Throws. 

Stealer of Souls. Whenever Aabaloth reduces a creature to 0 HP, he regains 50 Hit Points. Any healing that exceeds his Hit Point Maximum in this way is instead converted into Temporary Hit Points. 

Highly Reactive. Aabaloth can take one Reaction on every turn of combat and has 3 Bonus Actions on his turn. 

Regeneration. Aabaloth regains 30 Hit Points at the start of each of his turns if he has at least 1 Hit Point remaining.

Undying. When Aabaloth is reduced to 0 HP, he does not die. Instead, he recovers a new body in 1d100 days, restoring himself completely. 

Unusual Nature. Aabaloth does not require air, food, drink, or sleep. 

Multiattack. Aabaloth can make four Eldritch Bursts per turn and then use either Flight of the Damned, Rotten Fate, Curse of Withering Doom, or Summon Undead Legions if any are available. He can replace Eldritch Burst with any combination of attacks using Afterthought or Second Thought. 

Eldritch Burst. Melee or Ranged Attack Roll: +18 to hit, reach 5 ft. or range 120 ft. Hit: 41 Force damage and the target receives the Prone condition. 

Afterthought. Melee Weapon Attack: +14 to hit, reach 5 ft. Hit: 13 Piercing damage and 9 Psychic damage. The target must succeed a DC 22 Constitution Saving Throw or become Stunned for 1 turn. 

Second Thought. Melee Weapon Attack: +14 to hit, reach 5 ft. Hit: 15 Piercing damage and 10 Psychic damage. This attack lands guaranteed Critical Hits against Stunned creatures. 

Flight of the Damned (Recharge 5-6). DC 25 Dexterity Saving Throw: all creatures within 120 ft. cone are assaulted by a torrent of flying, spectral entities. Fail: 30 Necrotic damage, 30 Psychic damage, and the affected creature is Frightened for 1 turn.

Rotten Fate. DC 25 Constitution Saving Throw: one creature within 120 ft. Fail: 65 Necrotic damage, 40 Poison damage, and the target has the Poisoned condition for 2 turns. A Humanoid killed by this attack is raised as a Zombie (*See Monster Manual) that is under Aabaloth's direct control, following any orders and instructions that Aabaloth gives it. 

Curse of Withering Doom. DC 25 Constitution Saving Throw: up to 3 targets within 60 ft. Fail: 40 Necrotic damage, 40 Poison damage, and the targets have Disadvantage on all Constitution checks and Saving Throws they make until their next Long Rest. 

Summon Undead Legions (Recharge 5-6). Aabaloth conjures Undead creatures whose combined average Hit Points do not exceed 400. These loyal minions serve directly under Aabaloth, following any orders and instructions that he gives them. 

Spellcasting. Aabaloth casts one of the following spells using Intelligence as his Spellcasting Ability (Spell Save DC: 27)

At-Will: Detect Magic, Detect Thoughts, Dispel Magic, Counterspell, Shield, Fireball (Level 7 version), Lightning Bolt (Level 7 version), Greater Invisibility, Mage Hand, Scrying

3/Day each: Create Undead (Level 9 version), Dimension Door, Plane Shift (self only)

2/Day each: Chain Lightning (Level 8 version), Finger of Death, Power Word Kill, Dominate Monster

Vile Step. Aabaloth teleports to any unoccupied area he can see within 60 ft. Each creature within 10 ft. of the area he left takes 13 Psychic damage.

Spirit Drain. DC 23 Constitution Saving Throw: Up to 4 targets within 60 ft. Fail: 20 Necrotic damage and Aabaloth regains Hit Points equal to the total amount of damage dealt. 

Rise of the Undead Legions. Aabaloth conjures 3 Skeletons, 4 Zombies, 2 Ghouls, and 1 Ghast that directly follow his commands, obeying all orders and instructions that he gives them. 

Fell Rebuke. In response to being hit by an attack, Aabaloth utters a fell word, dealing 14 Necrotic damage to the attacker. Then he teleports up to 30 feet to an unoccupied space he can see.

Life Disruption. When Aabaloth takes damage, he can use his Reaction to unleash a blast of negative energy. All creatures within a 20 ft. Emanation centered on Aabaloth must succeed a DC 15 Constitution Saving Throw or be reduced to 0 HP. 

Terrifying Gaze. After Aabaloth takes damage, he stares intensely at one creature within 30 ft. that he can see. That creature must succeed a DC 20 Wisdom Saving Throw or take 22 Psychic damage and become Frightened for 1 turn.

Legendary Action Uses: 3. Immediately after another creature's turn, Aabaloth can expend a use to take make one of the following abilities, regaining all expended uses at the start of his turn.

Grim Destruction. One creature within 60 ft. of Aabaloth must succeed a DC 17 Dexterity Saving Throw or be reduced to 0 Hit Points. 

Chill of the Looming Grave. One creature within 30 ft. of Aabaloth must succeed a DC 22 Constitution Saving Throw or take 30 Necrotic damage, 20 Cold damage, and have their Hit Point maximum decreased by 10 until their next Long Rest. 

Reaper's Strike. Aabaloth moves up to his movement speed and makes one attack using Afterthought or Second Thought against a creature within 5 ft. of himself. 

Eldritch Annihilation. Aabaloth makes one Eldritch Burst attack.