Urgoros, The Empty One

Mount. If Urgoros isn't mounted, it can use a bonus action to magically teleport onto his gloamwing mount, provided Urgoros and the gloamwing are on the same plane of existence. When it teleports, Urgoros appears astride the gloamwing along with any equipment he is wearing or carrying. While mounted and not incapacitated, Urgoros can't be surprised, and both him and his mount gain advantage on Dexterity saving throws.

Soulless Mind. Urgoros has resistance to soul damage.

Multiattack. Urgoros may use his Whispering Keen and then makes two scythe attacks. If the total damage dealt reduces at least one target to 7 or less hit points, Urgoros has resistance to psychic damage and advantage on Wisdom, Constitution, and Intelligence saving throws until the end of his next turn.

Scythe. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 11 (2d6 + 4) slashing damage + 14 (4d6) psychic damage. 

Whispering Keen (Recharge 4-6). Urgoros unleashes a swath of hideous whispers and screeches around him. Each non-Undead within 120 feet of him that can hear him must succeed on a DC 16 Constitution saving throw or take 5 (1d10) necrotic damage and have disadvantage on Wisdom and Constitution saving throws until the end of Urgoros's next turn.

Mind Twist (Recharge 5-6). Urgoros magically emits psychic energy in a 60-foot cone. Each creature in that area must succeed on a DC 16 Wisdom saving throw or take 26 (4d12) psychic damage and be stunned for 1 minute. The stunned creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Reap Soul (1/Day). Urgoros targets a creature he can see within 120 feet of him that has 2 or less hit points. That creature must succeed on a DC 20 Constitution saving throw or automatically die as its soul is torn from its body, causing its body to fall to the ground as a lifeless husk.

Urgoros can take 2 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature's turn. He regains spent legendary actions at the start of its turn.

Reveal. Urgoros makes a Wisdom (Perception) check or a Charisma (Insight) check.

Scythe. Urgoros makes a scythe attack.

Teleport (Costs 2 Actions). Urgoros and his gloamwing mount, along with all equipment they are wearing or carrying, teleport to an unoccupied space within 80 feet of them. The gloamwing may then move up to half its speed.