I was there when the old magic was written. Xerachiel is as old as the stars. Any magic below 4th level has no effect on him.

 Elemental Spellcasting. Xerachiel's spellcasting ability is Charisma (spell saving throw DC 24). He can innately cast the following spells requiring no material components:
3/day (acid): tidal wave
3/day (cold): ice storm
3/day (fire): fireball
3/day (lightning): lightning bolt
3/day (thunder): shatter (as a 4th-level spell)
1/day (acid): maelstrom
1/day (cold): freezing sphere
1/day (fire): wall of fire
1/day (lightning): storm sphere (as a 5th-level spell)
1/day (thunder): whirlwind
1/day (radiant): dawn
1/day (void): Void Bomb
1/day (All): Lucifer's Embrace

Multi Attack. Xerachiel is able to make 4 attacks with Eternal Damnation.

Slash. Melee Weapon Attack: +14 to hit, reach 10ft., 1 target. Hit: (4d10 + 14) [slashing] damage. 

Memetic Muscle Memory - Once an attack or special ability has been done against you, you can replicate it in near perfect detail as an action. If it is a spell and you have not spell slots, it uses one of the charges from the blade itself. If an ability or attack is replicated 3 times, the player permanently learns that ability 

Blade of Black Flames - You are able to send an arch of black flames hurtling at your opponent with a range of 30 feet. Make a melee attack roll, if it hits, target takes the normal 4d10 slashing damage, as well as 1d10 fire damage and 1d10 necrotic damage. Bypasses resistances. Think Ichigo's black getsuga tenshou for visual aid

All Your Sins Laid Bare - Choose three creatures that are able to see Eternal Damnation. These creatures must make a wisdom saving throw DC 20, or be tortured by all their past mistakes and regrets, taking 4d12 psychic damage.

Eternal Damnation - The Seraphium has fully awoken, and brings all its ability to bear. It's base damage is upped to a 4d12. On a successful attack, attempt to absorb the soul of the target into Eternal Damnation, where it is chained and bound to the will of the wielder. Creatures with a CR 0-6 Wis DC 20, CR 6-12 Wis DC 15, CR 12 + Wis DC 12. Players must make a Wis DC 18. 

Purge - As a player's entire turn, they can plunge Eternal Damnation into the ground, and purge every soul absorbed into it, bringing them into the battle. After the battle is finished, every soul is released to pass on to its afterlife. 

Hellfire - With a flick of his wrist, Xerachiel summons a pillar of hellfire on a creatures location. Target must make a Dex save DC 20 or take 4d12 fire damage that bypasses resistances.

Reality Alteration. Xerachiel uses his slight command over the powers of the cosmos and alters his reality only slightly, causing half of the damage he had taken to have never landed. Once an encounter.

Forgiveness. Xerachiel lets go of the transgressions committed against him, gaining confidence in himself. Gains advantage on his next action.

Disrespect. Xerachiel tires of the lack of manners. He makes a melee attack against a single target, giving them a backhanded slap. deals 1d6 bludgeoning. Target loses its reaction.

Scintilla. Xerachiel uses his lifetimes of skill with Eternal Damnation to stop an attack and counter with blinding speed, pushing the target back 20 ft. Target must roll a DC 20 Con check or be stunned for a round. Roll a D6 recharge on a 5 or 6.

You Dare?! Xerachiel punishes a successful attack with a black flame blade attack.

Willful Gaze. Xerachiel looks at a creature, causing all of its self preservation instincts to kick in. Creature must make a Wis saving throw DC 20. On a failed save, they stop mid attack, and must back away up to half of their movement.

Fallen. Xerachiel reveals himself as a fallen angel, sprouting a pair of black feathered wings and revealing his true ashen complexion with horns. Transforming like this heals him for 50 HP. He gains a +2 to AC

Concentrated Fury (Available after performing Fallen). Xerachiel uses his blinding speed to make 8 consecutive cuts in an instant with his black flames, dealing 12D12 of either slashing, fire or necrotic damage.

Torrent of Black Hellfire - The hate that Xerachiel has worked endlessly to control burns in his eyes. He levels his hand at his enemy and a torrent of black hellfire spews forth. Every creature in a 30 ft cone makes a Dex saving throw. On a failed save, they take 4d10 fire damage. On a successful save take half damage. This attack bypasses resistance and treats immunity as normal resistance. Creatures max HP is reduced by half of the damage dealt to it.

Mercy (Available after performing Seraphim). Xerachiel appears in front of the target, the image of an angel sent from heaven, as heavenly light shines down on him and the target. He places one finger on the targets forehead, uttering "Pater miserere animae huius." (Father have mercy upon this soul). Target must make a wisdom saving throw DC 20. On a failed save, the target is placed into a comatose "stunned state" They are unable to act or move, and must repeat the saving throw on each of their turns to wake from it. This will not reduce their hp to 0, only render them unable to act. Xerachiel catches them and guides them carefully to the ground. On a successful save, has no effect.

Dawnbreaker (Available after performing Seraphim). Xerachiel combines the dawn and sunburst spells. A column of brilliant sunlight 30 ft in radius falls from the heavens centered on Xerachiel. each creature in it must make a Constitution saving throw, taking 4d10 radiant damage on a failed save, or half as much damage on a successful one. A creature must also make this saving throw whenever it ends its turn in the cylinder. Being touched again with the light of heaven, Xerachiel unleashes his raw celestial power. Brilliant sunlight flashes in a 60-foot radius centered on Xerachiel. Each creature in that light must make a Constitution saving throw. On a failed save, a creature takes 12d12 radiant damage and is blinded for 1 minute. On a successful save, it takes half as much damage and isn't blinded by this spell. Undead and oozes have disadvantage on this saving through

Untold Horrors (Available after performing Arch Devil of Fury). Xerachiel bores into the surrounding minds, and shows them a thousand horrors of his hellish realm, including people screaming as they are skinned alive, burning in inextinguishable flames and the raw brutality mortals visit upon each other in raw unabated fury. Every sentient thing in a 30 ft radius of Xerachiel must make a DC 24 Wis saving through, or take 12D10 psychic damage and have disadvantage on attacks and saving throws for one round. On a successful save targets take half as much damage and don't have disadvantage.

Flame Shield (Available after performing Arch Devil of Fury). Xerachiels power manifests as black flames that half the damage of the incoming attack, physical or magical, and lashes out, dealing the other half of that damage to the attacker as fire or necrotic damage.

Relentless Fury (Available after performing Arch Devil of Fury). Xerachiel throws everything into the next attack, abandoning defense. His AC lowers by 4, but he launches all four of his attacks continuously for the next round against a single target. this means that on every players turn, he launches 4 attacks against his target. If his target is not unconscious by the end of the round, Xerachiel is winded and loses his action on his next turn. If the target does go unconscious, Xerachiel enters bloodlust.

Bloodlust (Available after performing Arch Devil of Fury). Xerachiel's fury knows no limits and he recognizes neither friend nor foe. Only when all the blood has been spilled and his enemies lay broken before him will his reason return. Xerachiel gains an attack for every creature within 50 feet of him. His speed is doubled during this time. Lasts for 6 rounds.

Using any of these mythic actions causes the area to grow dark, the sheer amount of power released destroying and altering the area, leaving small fires dotting the area as the only source of light. 

Seraphim. Xerachiel reveals himself as a Seraphim, the highest and most powerful tier of angelic beings. "I am Xerachiel Dawnbreaker, Keeper of the Sacred Tomes of Absolution, Guardian of the Pearly Gates" Xerachiel now how six black wings, two wrapped around his waist and two pressed against his back, imitating a robe, while his main wings remain useable for flight. This transformation adds 100 HP to his maximum and heals him to full health. Gains +2 AC

Arch Devil of Fury. Xerachiel reluctantly releases his full terrifying power into the world, resuming his form as an Arch Devil. He maintains his six wings, now batlike. His eyes glow an intense red. "I have wrought pain and unfathomable horrors the like of which would drive Gods mad. When your bodies burn to ash, and your souls arrive before your god shattered and corrupted, they will tremble and know you were sent by the Arch Devil of Fury!" This transformation gives Xerachiel another 100 to his maximum HP, and heals him to full health. Gains +2 AC