Grim Aura. Any hostile creature that ends its turn within 30 ft of the Master Necromancer must succeed on a DC 20 Constitution saving throw or take 22 (4d10) necrotic damage or half as much on a successful save. 

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

Animate Dead. The Master Necromancer can raise any corpse that is Huge or smaller of a monstrosity, humanoid, or beast with challenge rating 9 or lower. It keeps its orginal abilities and attacks, but loses any resistances, immunities, legendary actions and senses it had. It gains the immunities of its type, skeleton or zombie, and advantage on Wisdom saving throws against Turn Undead attempts. The Master Necromancer can issue mental commands to the creature as the animate dead spell. The Master Necromancer may use this feature on an existing undead to reassert its control over it. The Master Necromancer can control up to 4 animated dead at one time. 

Spellcasting. The Master Necromancer is a 20-level spellcaster. Its spellcasting ability is Charisma (spell save DC 20, +12 to hit with spell attacks). The Master Necromancer has following Wizard spells prepared:
Cantrips (at will): Chill Touch, Ray of Frost, Toll the Dead, Infestation
1st level (4 slots): False Life, Hideous Laughter, Ray of Sickness, Shield

2nd level (3 slots): See Invisibility, Invisibility

3rd level (3 slots): Counterspell, Life Transference

4th level (3 slots): Blight, Dimension Door

5th level (3 slots): Enervation, Negative Energy Flood

6th level (2 slots): Circle of Death

7th level (2 slots): Finger of Death

8th level (1 slot): Maddening Darkness

9th level (1 slot): Psychic Scream

Multiattack. The Master Necromancer makes two death's scythe attacks. 

Death's Scythe. Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 9 (2d4 + 4) slashing damage. In addition, the target must succeed on a DC 20 Constitution saving throw or suffer one level of exhaustion. If a creature is killed using this ability then its soul is trapped within the Death's Scythe.

Master Necromancer 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. Master Necromancer regains spent legendary actions at the start of his turn.

Siphon (Costs 2 Actions). Melee Spell Attack: +12 to hit, reach 5 ft., one living target. Hit: 23 (4d8+6) necrotic damage and gains temporary heath points equal to the damage done. If the target dies from the attack then the Master Necromancer gains double the temporary heath points.

Misty Step (Costs 1 Action). The Master Necromancer casts Misty Step.

Cast Spell (Costs 1 Action). The Master Necromancer casts a spell of 3rd level or lower.

Command Dead (Costs 1 Action). The Master Necromancer commands one of its animated dead to use its reaction to make one attack.