Aeristoth, Deathbringer

Innate Spellcasting. Aeristoth's innate spellcasting ability is Charisma (spell save DC 18, +10 to hit with spell attacks). It can innately cast the following spells, requiring no material components:

At will: detect thoughts, freedom of movement, vicious mockery (4d4 psychic damage)

3/day each: Confusion, Bane, mirror image, Banishing Smite

1/day each: Meteor Swarm, Dominate Monster, Feeblemind

Shadow Stealth. While in dim light or darkness, Aeristoth can take the hide action as a bonus action.

Sunlight Weakness. While in sunlight, Aeristoth has disadvantage on attack rolls, ability checks, and saving throws.

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

Multiattack. Aeristoth makes two Greatsword attacks.

Aetherius, Greatsword. Melee Weapon Attack: +11 to hit, reach 5 ft., one target. Hit: 12 (2d6 + 6) slashing damage; plus 5 (1d8) necrotic damage.

Horrifying Presence. Each non-undead creature in 60 feet of Aeristoth that can see it must succeed on a DC 15 wisdom saving throw or be frightened for 1 minute. A frightened target can repeat the saving throw at the end of each of its turns, with disadvantage if Aeristoth is in line of site, ending the effect on itself on a success. If a target's saving throw is successful or the effects ends for it, the target is immune to Aeristoth's Horrifying Presence for the next 24 hours.

Parry. Aeristoth adds 5 to its AC against one melee attack that would hit it. To do so, Aeristoth must see the attacker and be wielding a melee weapon.

Cleave.  When Aeristoth makes an attack on a target, he can use his reaction to make the exact same roll on another target within 5 ft. 

Aeristoth 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. The Aeristoth regains spent legendary actions at the start of its turn.

Attack. Aeristoth makes a weapon attack.

Hide: Aeristoth takes the hide action

Cast a Spell (Costs 2 Actions). Aeristoth casts one of his innate spells.

Shifting Nightmare (Costs 3 Actions). Aeristoth rapidly takes the form of several nightmarish creatures, lashing out at all nearby. Each creature within 10 feet of Aeristoth must succeed on a DC 16 Dexterity saving throw or take 18 (4d8) damage of a type chosen by Aeristoth: acid, cold, fire, lightning, or necrotic.