Avatar of Nerull

Legendary Resistance (3/Day). If Nerull fails a saving throw, he can choose to succeed instead.

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

Unturned. Nerull is immune to effects that turn undead. Additionally, any undead of his choice within 60 feet of Nerull have advantage on their saving throw against features that turn undead.

Grave Aura. Any creature within 60 feet of Nerull of his choice cannot regain hit points.

Draining Aura. Any creature that starts its turn within 60 feet of Nerull must make a DC 15 Constitution Saving Throw or take 1 level of exhaustion. A creature who makes a successful saving throw is immune to this effect for 24 hours. 

Reaper. Nerull captures the soul of any creature that he kills. These creatures cannot be brought back to life via magic short of a Wish spell or similar magic. If the Reaper special ability of Nerull is disabled, these creatures souls are released, and are able to be brought back to life by normal means.

Immutable Form. Nerull's physical form cannot be changed against his will.   

Spellcasting. Nerull is a 20th-level spellcaster. His spellcasting ability is Intelligence (spell save DC 24, +16 to hit with spell attacks). Nerull has the following spells prepared:

At will: detect magic, hold person

Cantrips (at will): chill touch, mage hand, prestidigitation, toll the dead 

1st level (4 slots): ray of sickness, shield, thunderwave

2nd level (3 slots): mind spike, misty step, ray of enfeeblement

3rd level (3 slots): bestow curse, counterspell, dispel magic, fireball

4th level (3 slots): blight, phantasmal killer

5th level (3 slots): dominate person, enervation, negative energy flood

6th level (2 slots): circle of death, disintegrate, globe of invulnerability, mental prison, soul cage

7th level (2 slots): finger of death, forcecage, power word pain

8th level (1 slot): Abi-Dalzim's horrid wilting, feeblemind, power word stun

9th level (1 slot): imprisonment, meteor swarm, power word kill  

Multiattack. Nerull uses one of his actions or spells and makes one attack with Lifecutter.

Lifecutter. Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 29 (6d6 + 8) slashing damage, plus 21 (6d6) necrotic damage. All creatures of Nerull's choice within 10 feet of a creature he hits with Lifecutter must make a DC 20 Constitution saving throw. A creature takes 13 (2d12) force damage on a failed save, or half as much on a successful one. 

Death's Grasp. Chilling energy springs forward from Nerull, wrapping itself around any 3 creatures of Nerull's choice. These creatures must make a DC 24 Strength saving throw or be pulled to an unoccupied space within 5 feet of Nerull. They also take 18 (4d8) cold damage. On a successful save they are not moved and take half damage.  

Afterlife Bolt. Ranged Weapon Attack: +16 to hit, range 150 ft., 1 target. Hit: If the creature has 100 hit points or fewer it must make a DC 15 Constitution saving throw or die. On a successful saving throw, or if the creature has more than 100 hit points, it instead takes 15 (3d8 + 8) radiant damage plus 27 (6d8) necrotic damage.

Targeted Death (Recharge 5-6). Nerull chooses one creature he can see within 60 feet of him. All attacks against that creature have advantage until the start of Nerull's next turn. 

Nerull can take 4 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. Nerull regains spent legendary actions at the start of his turn.

Call the Dead (1 Action). Nerull summons 1d6 zombies or skeletons from the battle around you to join him. 

Lifecutter (1 Action). Nerull makes an attack with Lifecutter.

Grave Action (2 Actions). Nerull takes either the Death's Grasp or Afterlife Bolt action. 

Cast a Spell (2 Actions). Nerull can cast any spell of 4th level or lower. 

Command Undead (2 Actions). Nerull commands up to 3 undead creatures he can see within 60 feet of him. The target can make one weapon attack as a reaction and gains advantage on the attack roll.

Darkness Rises (3 Actions). Each non-undead creature within 30 feet of Nerull must make a DC 24 Constitution saving throw, taking 21 (6d6) necrotic damage on a failed save, or half as much damage on a successful one. On a failed save, a targets maximum hit points are reduced by an equal amount of damage that they took.