Avatar of the Dark Lord

Legendary Resistance (3/Day). If the Avatar fails a saving throw, it can choose to succeed instead.

Aura of Unlight. A 30-foot-radius field of suffocating shadow surrounds the Avatar. Nonmagical light is extinguished. Magical light sources illuminate only half their normal radius. Any creature that starts its turn in the aura must make a DC 18 Wisdom saving throw or suffer disadvantage on attack rolls until the start of its next turn as despair gnaws at their will.

Umbral Regeneration. At the start of its turn, if the chamber’s corrupted crystal still glows faintly with shadow energy, the Avatar regains 20 hit points. When the crystal is purified or shattered, this trait ends.

Heart of the Void. Whenever the Avatar takes radiant damage, it explodes with backlash: each creature within 10 ft. of it must succeed on a DC 18 Constitution saving throw or take 10 (3d6) necrotic damage.

Multiattack. The Avatar makes two Claws of Oblivion attacks.

Claws of Oblivion. Melee Weapon Attack: +11 to hit, reach 10 ft., one target.
Hit: 27 (6d8 + 6) necrotic damage.

Tendrils of the Abyss (Recharge 5–6). Tendrils of black ether erupt from cracks in the floor in a 30-ft. radius around the Avatar. Each creature must make a DC 18 Strength saving throw, taking 36 (8d8) necrotic damage on a failed save, or half on a success. A creature that fails is also restrained until the start of the Avatar’s next turn.

Withering Howl (Recharge 4–6). The Avatar releases a psychic scream that echoes through the sanctum. Each creature within 60 ft. must make a DC 18 Wisdom saving throw or take 32 (7d8) psychic damage and be frightened until the end of its next turn. On a success, they take half damage and are not frightened.

Summon Shadows (1/Day). The Avatar summons 2d4 shadows that crawl out from fissures in the walls or emerge from the players’ own shadows. These act on initiative count 10 and vanish when the Avatar is destroyed.

The Avatar can take 3 legendary actions, only one at a time. It regains all at the start of its turn.

Move. The Avatar moves up to 30 ft. without provoking opportunity attacks.
Grasping Shade. One creature within 15 ft. must make a DC 18 Dexterity saving throw or take 13 (3d8) necrotic damage and be pulled 10 ft. closer to the Avatar.
Siphon Life. The Avatar targets one creature within 10 ft. The target must make a DC 18 Constitution saving throw or take 18 (4d8) necrotic damage, and the Avatar regains half as many hit points.
Shadow Rift (Costs 2 Actions). The Avatar creates a 10-ft.-radius rift in space at a point it can see within 60 ft. The area becomes difficult terrain and heavily obscured until the start of the Avatar’s next turn; any creature that starts its turn inside takes 10 (3d6) necrotic damage.