Be'lakor, the Dark Master

Daemonic. Be'lakor has advantage on saving throws against spells and magical effects. When Be'lakor dies, his body and equipment disintegrates, leaving nothing behind. 

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

Magic Weapons. Be'lakor's weapon attacks are magical.

Lord of Torment. When an enemy creature starts its turn within 30 feet of Be'lakor, it must succeed on a DC 23 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. A creature that succeeds on this saving throw is immune to Be'lakor's Lord of Torment effect for 24 hours. If a creature that is frightened dies while within 30 feet of him, Be'lakor regains hit points equal to half the creature's hit point maximum.

The Dark Master. Demons that are within 30 feet of Be'lakor and are friendly to him have advantage on saving throws against being charmed or frightened. 

Psyker. Be'lakor's spellcasting ability is Charisma (spell save DC 23). Be'lakor can innately cast the following spells, requiring no material components. 

At will: detect magic, darkness

3/day each: dispel magic, shadow of moil, phantasmal killer

1/day each: negative energy flood, maddening darkness

Multiattack. Be'lakor makes two attacks with his Blade of Shadows and one attack with his Spearing Tail. He can replace any number of these attacks with a Smite attack. 

Blade of Shadows. Melee Weapon Attack: +17 to hit, reach 10 ft., one target. Hit: 30 (6d6 + 9) slashing and 13 (3d8) necrotic damage. If the target is frightened, Be'lakor has advantage on the attack roll. 

Spearing Tail. Melee Weapon Attack: +17 to hit, reach 20 ft., one target. Hit: 20 (2d10 + 9) piercing damage. 

Smite. Ranged Spell Attack: +15 to hit, range 120 ft., one target. Hit: 22 (4d10) lightning damage. 

Shadow Form. While in dim light or darkness, Be'lakor teleports up to 120 feet to an unoccupied space he can see that is also in dim light or darkness. 

Be'lakor 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. Be'lakor regains spent legendary actions at the start of his turn.

Attack. Be'lakor makes a melee attack. 

Shrouded Step. Be'lakor teleports to an unoccupied space he can see within 120 feet. 

Cast Spell (costs 2 actions). Be'lakor casts one spell from his Psyker trait.

Voidslivers (costs 3 actions). Be'lakor chooses up to three creatures he can see that are frightened. Those creatures must make a DC 23 Wisdom saving throw, suffering 26 (4d12) psychic damage on a failed save, or half as much on a successful save.