Legendary Resistance (8/Day). If Symforá fails a saving throw, he can choose to succeed instead.

Magic Weapons. Symforá's weapon attacks are magical.

Umbral Awakening (Recharges after a Short or Long Rest). If Symforá would be reduced to 0 hit points, his current hit point total instead resets to 1000 hit points, he recharges his Sinful Smite, and he regains any expended uses of Legendary Resistance. Additionally, Symforá can now use the options in the “Mythic Actions” sections, adding 3 actions to the maximum. Award a party an additional 155,000 XP (310,000 XP total) for defeating Symforá after his Umbral Awakening activates.

Special Equipment. Symforá wields his own weapon called Symforá's Infernal Whip.

Reactive. Symforá can take one reaction on every turn in a combat.

Magic Resistance. Symforá has advantage on saving throws against spells and other magical effects meant to harm him.

Devil's Sight. Magical Darkness doesn't impede Symforá's darkvision.

Fear Aura. Any beast or humanoid with an intelligence above 5 that starts its turn within 10 ft. of Symforá must succeed on a DC 17 Wisdom saving throw or be frightened until the start of the creatures next turn.

Regeneration. Symforá regains 50 hit points at the start of its turn if it has at least 1 hit point. 

Immortal. Unless sealed away, Symforá will always come back within 1d10 days, being revived in a random location 30 ft. from where he was killed.

Innate Spellcasting. Symforá's innate spellcasting ability is Charisma (Spell save DC 30, +25 to hit). Symforá can cast the following spells without needing components or expending a spell slot:

At will: animate dead, geas (9th level), ice storm (9th level), thunderwave (9th level), fire bolt (max level), chill touch (max level), bestow curse (9th level), fireball (9th level), chain lightning (9th level), disguise self, identify, blight (6th level), phantasmal force
4/day each: blindness/deafness (9th level), enthrall, dispel magic (9th level), counterspell (9th level), slow (unlimited range), hold monster, mass suggestion (9th level), wall of fire, wall of ice, sickening radiance, polymorph
3/day each: banishment, phantasmal killer, dominate person (9th level), mental prison, harm, finger of death, forcecage, divine word, greater invisibility, symbol
2/day each: feeblemind, dominate monster (9th level), shapechange, imprisonment, meteor swarm, power word kill, true polymorph, mass polymorph
1/day each: dark star, true resurrection

Multiattack. Symforá makes 3 attacks: 2 with his claws, and one with his darkening grasp. He can replace one of these attacks with a use of his infernal whip.

Infernal Whip. Melee Weapon Attack: +32 to hit, reach 10 ft., 1 target. Hit: 60 (1d4 + 5d12 + 5) slashing damage + 12 (1d10 + 5) poison damage + 14 (1d12 + 5) fire damage.

Shadowy Shroud. Smoke vapor rotates around Symforá, devouring him. Anyone who wishes to see him must make a DC 19 Perception, or Investigation check. One he is seen, his cover is blown, and the shadows evaporate. These shadows last until the end of his next turn. While he is shrouded, his Darkening Grab does an extra 1d4 necrotic damage.

Darkening Grasp. Melee Weapon Attack: +10 to hit, reach 10 ft., 1 target. Hit: 23 (1d10 + 20) necrotic damage + 4 (1d6) poison damage + 5 (1d8) fire damage.

Claw. Melee Weapon Attack: +10 to hit, reach 5 ft., 1 target. Hit: 14 (1d6 + 10) slashing damage + 14 (4d6) fire damage.

Change Shape. Symforá magically transforms into any creature that is below CR 25, while retaining his game statistics (other than its size). This transformation ends if Symforá is reduced to 0 hit points once or uses a bonus action to end it. He can choose to transform all of his gear, some of it, or none of it.

Elemental Bane. Symforá summons one elemental below CR 12 within 30 ft. of him. This elemental curses one creature it can see, devoting its life to hunting down and killing that creature unless commanded to do otherwise.

Psychic Step. Symforá magically teleports to an unoccupied space he can see within 60 feet of it.

Invisibility. Symforá magically turns invisible until he attacks or until his concentration ends (as if concentrating on a spell). Any equipment Symforá wears or carries is invisible with him.

Shadow Shield. Symforá creates a pentagram of shadows, adding 5 to his AC against the attack that triggers this reaction.

Umbral Thrall. When Symforá is hit by an attack roll, one creature within 20 feet of him that is charmed or frightened by him is hit by the attack instead.

Symforá can take 5 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. Symforá regains spent legendary actions at the start of his turn. His mythic actions and legendary actions share the same 5 actions.

Infernal Whip (Costs 3 Actions). Symforá uses his Infernal Whip.

Spellcasting (Costs 2 Actions). Symforá uses spellcasting or he uses Psychic Step, Invisibility, Elemental Bane, or Change Shape.

Shove. One target within 5 ft. of Symforá makes a DC 19 Dexterity saving throw. On a fail they are pushed back 10 ft. and knocked prone. On a success they are only pushed back 10 ft.

Regain. Symforá regains 30 hit points.

Shadow Spin. Symforá generates a spinning pentagram of shadows, summoning either 2 shadows, 2 shadow mastiffs, 1 shadow mastiff alpha, 1 shadow demon, or 1 wraith within 10 ft. of him.

Symforá can take 5 mythic actions, choosing from the options below. Only one mythic action option can be used at a time and only at the end of another creature’s turn. Symforá regains spent mythic actions at the start of his turn. His mythic actions and legendary actions share the same 5 actions.

Sinful Smite (Costs 5 Actions, can only be used once per life). Melee Weapon Attack: +20 to hit, reach 15 ft., 1 target. Hit: 200 (35d10) force damage. 

Infernal Whip (Costs 3 Actions). Symforá uses his Infernal Whip.

Spellcasting (Costs 2 Actions). Symforá uses spellcasting or he uses Psychic Step, Invisibility, Elemental Bane, or Change Shape.