Avatar of Dendar

Amorphous. The Avatar can move through a space as narrow as 1 inch without expending extra movement to do so.

Immutable Form: The Avatar is immune to any spell or effect that would alter its form.

Incorporeal Movement. The Avatar can move through other creatures and objects as if they were Difficult Terrain. It takes 5 (1d10) Force damage if it ends its turn inside an object.

Innate Spellcasting. The Avatar is an 20th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 28, +20 to hit with spell attacks) and can cast the following spells:

At Will: Dissonate Whispers, Mind Sliver, Viscous Mockery
4/day: Antagonize, Hunger of Hadar
3/day: Phantasmal Killer, Synaptic Static
1/day: Power Word Kill

Living Shadow. While in Dim Light or Darkness, the Avatar has Resistance to damage that isn’t Force, or Radiant.

Legendary Resistance (7/Day). If the aspect fails a saving throw, she can choose to succeed instead.

Your Worst Nightmare: Any Psychic damage dealt by the Avatar ignores Resistance and treats Immunity as Resistance. When the Avatar imposes the Frightened condition, targets cannot benefit from immunity to that condition. If they would be immune they instead add a +6 bonus to their Saving Throw.

Multiattack. The aspect makes 2 Shadow Cleave attacks, and one Nightmare Lance attack.

Shadow Cleave. Melee Weapon Attack: +19 to hit, reach 15 ft., one target. Hit: 36 (4d12 + 10) slashing damage plus 33 (5d12) force damage.

Nightmare Lance. Ranged Spell Attack: +20 to hit, reach 120/300 ft., one target. Hit: 30 (3d12 + 11) piercing damage plus 26 (4d12) Psychic damage. The target must make a DC 28 Wisdom saving throw, on a failure the target is frightened.

Shadow Wave (Recharge 5–6). The avatar unleashes a wave of dark despair in a 100-foot cone. Each creature in that area must make a DC 28 Dexterity saving throw. On a failed save, the creature takes 85 (13d12) psychic damage: psychic On a successful save, the creature takes half as much damage.

Shadow Step. While in dim light or darkness, the Avatar teleports, along with any equipment he is wearing or carrying, up to 60 feet to an unoccupied space he can see. He can cause each creature of his choice within 15 feet of his destination space to take 23 (5d8) psychic damage. If at least one creature takes this damage, the Avatar regains 80 hit points.

The Avatar can take up to three reactions per round but only one per turn.

Critical Exchange: Once per day, when hit with a critical hit, the Avatar turns the attackers success into a nightmare. All damage that would be done to the Avatar instead is dealt to another creature of the Avatar's choice within 100 ft.

Dread Counterspell. The Avatar utters a dread word to interrupt a creature he can see that is casting a spell. If the spell is 4th level or lower, it fails and has no effect. If the spell is 5th level or higher, the creature makes a constitution saving throw DC 28. On a failed save, the spell fails and has no effect. Whatever the spell’s level, the caster takes 18 (4d8) psychic damage if the spell fails

The Avatar can take a number legendary actions equal to the number of enemies it is facing each round, 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, and can only use Despair once per round. The aspect regains spent legendary actions at the start of her turn.

Attack. The Avatar makes one Shadow Cleave or Nightmare Lance attack. 

Cast. The Avatar casts a spell.

Despair (Costs 3 Actions). Creatures Frightened by the Avatar must make a DC 27 Wisdom Saving Throw or take 59 (9d12) Psychic Damage