Ashiok, Dream Render

True Sight. Range 120 feet.

Nightmare Weave (Mythic Trait; Recharges after Long Rest). If Ashiok is reduced to 0 hit points, the open nightmare portals close and coalesce into the corpse of Ashiok reviving them by their full amount of hit points regardless of penalties and granting an additional 100 temporary hit points. Aberrant spirit summoning ends at this point due to portal closures.

Legendary Resistance (3/Day). If Ashiok fails a saving throw, they can choose to succeed instead.

Magic Resistance. Ashiok has advantage on saving throws against spells and other magical effects.

Spellcasting. Ashiok is a 20th-level spellcaster. Its spellcasting ability is Charisma (spell save DC 23, +15 to hit with spell attacks). Ashiok has following Sorcerer spells prepared:
Cantrips (at will): [Minor Illusion], [Mind Sliver], [Mage Hand], [Dancing Lights], [Message], [Prestidigitation] 
1st level (at will): [Disguise Self], [Sleep], [Charm Person]

2nd level (at will): [Detect Thoughts], [Summon Shadow Spirit]

3rd level (at will): [Fear] 

4th level (3 slots): [Summon Aberrant Spirit], [Greater Invisibility]

5th level (3 slots): [Far Step], [Synaptic Static]

6th level (3 slots): [Mental Prison]

7th level (3 slots): [Finger of Death]

8th level (2 slots): [Abi-Dalzim's Horrid Wilting]

9th level (1 slot): [Wish]

Multiattack. Ashiok casts one spell and either uses their Invoke Nightmares ability or makes two claw attacks.

Claws. Melee Weapon Attack: +11 to hit, reach 5 ft., 1 target. Hit: 7 (1d6 + 4) slashing damage. If both claw attacks hit, the target creature's connection to their Theran god is severed and halves their current piety score. If piety is lowered below reward tiers, target creature loses access to those perks. Piety is restored at end of a long rest. 

Invoke Nightmares. Ashiok targets one creature they can see within 60 feet of them. The target must succeed on a DC 23 Wisdom Saving throw or be cursed by nightmares. Until the curse is ends, the target can't regain hit points and has vulnerability to necrotic damage. They also see their allies as potential threats. [Greater Restoration], [remove curse] or similar magic ends the curse on the target.

Ashiok 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. Ashiok regains spent legendary actions at the start of their turn.

At-will spell. Ashiok casts one of their at-will spells.

Detonate Shadows (Costs 2 Actions). Ashiok chooses two Aberrant or Shadow Spirits to explode in a 10 foot radius. Creatures within the blast take 18 (4d8) force damage.

Disrupt Life (Costs 3 Actions). Each creature within 20 feet of Ashiok must make a DC 20 Constitution saving throw against this magic, taking 42 (12d6) necrotic damage on a failed save, or half as much damage on a successful one.

If Ashiok's mythic trait is active, they can use the options below as legendary actions, as long as they have temporary hit points from their Nightmare Weaving.

Wish (Costs 3 Actions). Ashiok casts wish, replicating the effects of the simulacrum spell, targeting themself. The simulacrum does not have Ashiok's Nightmare Weave ability.

Collapse of the Nightmare Realm (Costs 2 Actions). 1d4 tiles of the dungeon map vanish. Creatures standing on these tiles must complete a DC 25 Dexterity saving throw, taking 13 (3d8) force damage on a failed save, or half as much damage on a successful one. On failure, the creature teleports to a random square on the map and the creatures are no longer able to move through them or end their turn on them.