Belial, Shadow-Priest King

Mythic Creature. Belial treats all attempted D20 Tests and damage rolls targeting & being targeted by non-epic creatures as its maximum value. 

Mythic Resistance. When Belial fails a Saving Throw, he may expend a legendary action to succeed instead.

Mythic Transformation (1/day). When Belial is reduced to 550 Hit Points, he immediately the Influence Action and the multiattack action, regains 300 Hit Points, and gains the following benefits until he takes a Long Rest: His Regal Regeneration improves by 20 Hit Points, he gains 1 additional Legendary action each round, his pleasure in pain activates when he takes 30 or more damage, his melee attacks deal an extra 11 psychic damage, effects he causes that charm no longer requires concentration and the duration is until Belial is reduced to 0 Hit Points. In additon, until the end of his next turn, psychic and fire damage he deals is doubled, creatures have Epic Disadvantage on saves against his effects that charm and they ignore condition immunity to being Charmed. 

Aura of Dread. Emmination 60 feet. WisSv:DC23. Failure: 16 psychic damage and has Epic Disadvantage on attacm rolls targeting Belial. Repeat the save & effect at the start of an effected creature's turn. Success: the effect ends and the creature is immune to this aura for 24 hours.

Regal Regeneration. Unless Belial has taken Radiant damage from Holy Magic or damage by a weapon forged in a Good-Alligned outer plane, he regains 30 hit points at the start of each of his turns. This regeneration functions at 0 Hit Points unless disabled. Disabling this regeneration lasts until the end his next turn.

Pleasure in Pain. Whenever Belial takes 50 damage from a single source, he may choose to regain up to half the damage taken and get a +1 to his next D20 Test.

Gift of Kings. When Belial casts a spell that requires concentration, the Septer of the Shadow-King may concentrate on it instead. The Septer can only concentrate on one spell at a time and Belial does not need to take concentration saves for this spell.

Infernal Wounds. Creatures damaged by Belial's melee attacks cannot regain hit points for 1 minute.

Sentient Hellfire. At the start of Belial's turn, each spell he or the Septer are concentrating on that deals fire damage animates to life. It no longer requires concentration & becomes a Living Hellfire dealing the spell's damage to creatures it passes with its movement. These Living Hellfires act on Inisiative 1. Living Hellfires summoned this way dispel after 1 minute.

Infernal Spellcasting. Belial is a 22nd-level spellcaster. Its spellcasting ability is Charisma (spell save DC32, +19 to hit with spell attacks). Belial has following spells prepared: (*spells require concentration)
At-will: Create Bonfire*(Tier-8), Mote of Hell*, Wall of Fire*, Mass Suggestion, Seeming, Major Image (4th-level), Plane Shift (Self)
3/day: Delayed Blast Fireball*, Wildfire*, Dominate Person*(8th-level), Dominate Monster*(8th-level), Demand*, Mirage Arcane, Project Image*
1/day: Bloodbane*, Caldera*, Determinism, Enigma Masque*, Hideous Mandate, Meteor Swarm

Multiattack. Belial makes up to 3 melee attacks then uses his Spellcasting or his Wall of Desire. Instead of Wall of Desire, he may use his Insanity Eruption if available.

Ranseur of Torture. Melee Weapon Attack: +21 to hit, reach 15 ft., 1 target. Hit: 36 Piercing damage plus 18 Psychic damage. A Hit creature must succeed a ConSv22 or has the paralyzed condition until the end of its next turn.

Wall of Desire. 100-foot-Long, 15ft-Wide Line: WisSv27, range 120 ft., creatures in the area. Failure: Charmed by Belial for 1 minute or until it succeeds a repeated save at the end of each of their turns.

Insanity Eruption (Recharge 5-6). 30-foot Emmination: IntSv27, range:self, creatures in the area. Failure: 55 Psychic damage and suffers the effect of the Confusion spell and gains short-term madness until the end of its next turn. Success: 24 Psychic damage only.

Infernal Curse. One visible creature that can hear Belial, WisSv27, Duration: Until Belial is reduced to 550 or 0 hit points (whichever is sooner). Failure: is cursed with one of the following effects:

• Lies of a Thousand Severed Tongues: You cannot take bonus actions or reactions.
• Whispers of a Screaming Choir: Gain vulnerability to psychic damage.
• Bliss of the Flayed Souls: Epic Disadvantage to save against being Charmed or Frightened. Lose Immunity to these effects from Belial.
• Spreading of Everburning Echos: Take 11 fire damage at the start of each of your turns and fire damage you take lowers your Hit Point Maximum by the same amount.
• Cacophony of Broken Minds: Take 11 psychic damage at the start of each of your turns and psychic damage you take results in your speed reducing to 0 until the start of your next turn.

Exquisite Agony. Trigger: creature within 60 feet succeeds a D20 Test; Effect: The creature must roll it again and take the lowest result. 

Bathe in Hellfire. Trigger: Belial is effected by a harmful condition; Effect: Belial teleports into a space occupied by a Living Hellfire he controls. He is immune to all damage except psychic until the Living Hellfire is reduced to 0 Hit Points or at the start of Belial's next turn.

Dark Infuence. Trigger: Belial is targeted by an attack, spell, or psionic; Effect: Belial takes the Infuence Action speaking only to the creature that triggered this. If he rolls lower than a 15, the dice result becomes a 15.

Legendary Action Uses: 3 (4 after Mythic Transformation). Immediately after another creature's turn, Belial can expend a use to take one of the following actions. Belial regains all expended uses at the start of each of their turns. 

Infernal Teleportation. The archdevil teleports themself to an area they can see within 120 feet. 

Ranseur. Belial moves up to his speed and makes 1 attack with his Ranseur of Torture.

King's Command (Costs 2 Actions). Creatures Charmed, have Madness because of, or are Incapacited by Belial takes 45 psychic damage and are stunned until the end of their next turn, alternatively effected targets may choose to instead make a multiattack action or Magic action to cast a spell of 5th level or higher targeting only allies as if they were hostile during their next turn. 

Scepter of Kings (Costs 1-3 Actions). Enter the description for your legendary action.The archdevil uses their Spellcasting feature to cast an At-Will spell. If the spell normally requires concentration, the Scepter must concentrate on it instead. For 2 Actions: a 3/day spell. For 3 Actions: a 1/day spell.

Call Underling (Costs 2 Actions). Belial summons a Pit Fiend under his control. It takes its turn on inisiative 20.

Belial takes 1 Mythic Action on Inisiative 20 each round. He may not choose the same effect two rounds in a row.

Unholy Seduction. One Visible Target that can hear Belial. The target suffers the effects of Dominate Monster until Belial does another Mythic Action.

Absolute Hellfire. Three Targets, range 120 feet. Whenever the creatures move they take 5 fire damage per 5 feet moved, in additon the targets lose resistance and immunity to fire damage. Both effects end when Belial does another Mythic Action.

Delight in Siffering. One Target, range visible. Whenever the creature damages a creature within Belial's visibility, he restores 30 Hit Points until he does another Mythic Action.