Amananoth, the Azure Thunderbrand

Astral-Skinned. Amananoth’s body is partially phased into the Astral Plane. It can move through objects and creatures as if incorporeal during its movement, though it cannot end its turn inside them. It is immune to grapple, restraint, and petrification.

Soulstatic Wake. When Amananoth moves through a creature’s space, it leaves behind a trail of soul-disrupting energy. At the end of its turn, each creature it passed through must succeed on a DC 22 Wisdom saving throw or take 4d10 psychic damage and lose concentration.

Threadveil Aura. A 30-foot aura of shimmering soul-threads surrounds Amananoth. Creatures that start their turn in the aura must succeed on a DC 21 Charisma saving throw or become Incorporeal until the end of their next turn (can’t interact with physical objects, can move through walls, but can’t cast spells or make attacks).

Spellcasting. Amananoth is a 17th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 22, +14 to hit with spell attacks). Amananoth has the following spells prepared:
Cantrips (at will): mage hand, minor illusion, thaumaturgy
1st level (4 slots): shield, disguise self
2nd level (3 slots): mirror image, detect thoughts
3rd level (3 slots): counterspell, nondetection
4th level (3 slots): greater invisibility, dimension door
5th level (2 slots): dream, telekinesis
6th level (1 slot): dispel magic, true seeing
7th level (1 slot): etherealness
8th level (1 slot): maze
9th level (1 slot): astral projection

Astral Bite. Melee Weapon Attack: +14 to hit, reach 10 ft., one target. Hit: 45 (10d6 + 10) piercing damage plus 18 (4d8) psychic damage.

Tail Veil. Melee Weapon Attack: +14 to hit, reach 20 ft., one target. Hit: 38 (6d10 + 10) force damage. The target must succeed on a DC 22 Wisdom saving throw or become Incorporeal until the end of its next turn.

Soul Rend (Recharge 5–6). Amananoth targets up to three creatures it can see. Each must make a DC 22 Charisma saving throw. On a failure, the creature takes 66 (12d12) psychic damage and cannot regain hit points until the end of its next turn.

Phase Step. Amananoth shifts partially into the Astral Plane and teleports up to 60 feet to an unoccupied space it can see. This movement does not provoke opportunity attacks.

Threadlash. When a creature within 30 feet casts a spell, Amananoth can lash its soul-thread. The caster must succeed on a DC 22 Intelligence saving throw or lose the spell and take 22 (5d8) psychic damage.

Amananoth can take 3 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.

Tail Veil. Amananoth makes a Tail Veil attack.

Phase Flicker. Amananoth teleports up to 30 feet.

Soulstatic Pulse (Costs 2 Actions). Each creature within 20 feet must succeed on a DC 22 Wisdom saving throw or take 27 (6d8) psychic damage and lose concentration.

Sever the Thread. Amananoth targets up to three creatures it can see. Each must make a DC 22 Charisma saving throw. On a failure, the creature’s astral form is forcibly projected for 1 minute (as astral projection, but the body remains stunned and vulnerable). If the astral form is slain, the body dies instantly.

Memory Collapse (Cost 2 Actions). Amananoth forces all creatures within 60 feet to make a DC 22 Intelligence saving throw. On a failure, the creature forgets its last turn and cannot take reactions or bonus actions until the end of its next turn.