Asharderis, King of Demons

Discorperation. When Asharderis drops to 0 hit points or dies, his body is destroyed but his essence travels back to the heart of the Endless Abyss, and he is unable to take physical form for a time.

Immortal. Asharderis cannot be destroyed by a single spell or effect, nor can he be entirely erased from existence, in addition, he is immune to any effects that would instantly reduce him to 0 hit points.

Imperious Garb. Draped in the Robes of the Scarlet Emperor, Asharderis has a +2 bonus to all saving throws he isn't naturally proficient in.
In addition, while he has more than half his hit points remaining, no attack roll or ability check can have advantage against him unless such is granted by other divine powers, racial abilities, or from a spell of 9th Level or higher. If sullied by holy oil or water, effects may gain advantage against him for 1 round.

Infernal Transcendence (Recharge After a Short or Long Rest). Asharderis sheds his demonic visage, revealing the twisted divinity within, corrupted and pulsing with power gifted by two of the Primoridal Gods, he falls upon his foes as a force of nature more than a monster. If Asharderis would be reduced to 0 hit points, his current hit point total instead resets to full, he regains any expended spells, and he regains expended uses of Legendary Resistance. Additionally, Asharderis uses his mythic stats for the next hour. The XP reward for defeating Asharderis doubles, after he utilizes Infernal Transcendence.

Innate Spellcasting. Asharderis' spellcasting ability is Charisma (spell save DC 27, +19 to hit with spell attacks). He can innately cast the following spells, requiring no material components:
At will: Bane, Burning Hands, Crown of Madness, Darkness, Detect Magic, Detect Thoughts, Dispel Magic, Fire Bolt (17th Level), Fireball, Hex, Hold Monster, Inflict Wounds (5th Level), Magic Missile, Prestidigitation, Thunderwave (4th Level)

5/day each: Clairvoyance, Command, Counterspell, Dimension Door, Elemental Weapon, Hunger of Hadar, Lightning Bolt, Mind Sliver (17th Level), Scorching Ray (4th Level)

4/day each: Banishment, Blight, Dispel Magic, Enemies Abound, Fear, Gaseous Form, Polymorph, Scrying, Silence

3/day each: Chain Lightning, Cloudkill, Cone of Cold, Divine Word, Greater Invisibility, Harm, Mass Suggestion, Plane Shift, Reverse Gravity, Scatter, Shatter, Wall of Fire

2/day each: Circle of Death, Control Weather, Creation, Delayed Blast Fireball, Disintegrate, Dominate Monster, Eyebite, Finger of Death, Fire Storm, Instant Summons, Power Word: Pain, Symbol, Teleport, Whirlwind

1/day each: Antimagic Field, Earthquake, Gate, Incendiary Cloud, Meteor Swarm, Mirage Arcane, Storm of Vengeance, Wish

King of Demons. All fiends with a Challenge of 20 or lower make attack roles against Asharderis with disadvantage.

Legendary Resistance (5/Day). If Asharderis fails a saving throw, he can choose to succeed instead.

Limited Magic Immunity. Asharderis is immune to spells of 5th level or lower unless he wishes to be affected. He has advantage on saving throws against all spells and magical effects.

Magic Weapons. Asharderis' weapon attacks are magical.

Regeneration. Asharderis regains 30 hit points at the start of his turn.

Supreme Spelltwister. When Asharderis casts a spell that deals damage, he can change that spell's damage to any damage type other than Psychic, Radiant, Bludgeoning, Piercing, or Slashing. He must do this before he casts the spell.

Tainted Weave. Asharderis can maintain concentration on two different spells at the same time. In addition, he has advantage on saving throws to maintain concentration on spells. As well, creatures hostile to Asharderis have disadvantage on concentration saving throws for spells that effect exclusively Asharderis.

Tithe of Souls. When Asharderis or a demon within 120 feet of him reduces a creature with a soul to 0 hit points, that creature automatically fails one of its death saves, and Asharderis gains 25 temporary hit points. In addition, all demons within 60 feet of Asharderis gain 10 temporary hit points, except if that specific demon is responsible, in which case they gain 15 temporary hit points. Asharderis can choose to exclude any creature from gaining aid from this ability.

Multiattack. Asharderis makes two melee attacks, either of which he can replace with a use of Crimson Demonflame, he may also cast a spell, or use one of his recharge abilities.

Soul Shattering Touch. Melee Weapon Attack: +15 to hit, reach 5 ft., one creature. Hit: 24 (4d8+6) necrotic damage, and the target must succeed on a DC 19 Constitution saving throw or its hit point maximum is reduced by an amount equal to the damage taken. This reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0.
Asharderis regains hit points equal to half the damage dealt.

Staff of the Endless Abyss. Melee Weapon Attack: +18 to hit, reach 10 ft., one target. Hit: 22 (2d12 + 9) bludgeoning damage plus 21 (6d6) psychic damage.
On a critical hit, the target must succeed on a DC 27 Constitution saving throw or be Stunned until the end of its next turn.

Crimson Demonflame. Ranged Spell Attack: +19 to hit, range 120 ft., one target. Hit: 35 (10d6) fire and necrotic damage.

Demon King's Judgement (Recharge 5-6). Asharderis unleashes a beam of demonflame in a 120-foot line that is 10 feet wide. Each creature in that line must make a DC 27 Dexterity saving throw, taking 49 (9d10) fire damage plus 49 (9d10) necrotic damage on a failed save, or half as much damage on a successful one.

Kneel Before Me (Recharge After a Long Rest). Each hostile creature within 30 feet of Asharderis must succeed on a DC 25 Intelligence saving throw, taking 52 (15d6) psychic damage on a failed save, or half as much on a successful one.
For each creature that fails the save, roll a d10 and apply the corresponding effect.
1- The target is Frightened for one round with no further consequences.
2- The target's alignment moves one step towards chaotic evil, and it gains more demonic feature. This change can only be undone using Greater Restoration or a similar spell at 7th Level or higher.
3- The target looses any damage resistances it has and any immunities it possessed instead become resistances. These effects last until the target completes a short or long rest, or until it's cured by Remove Curse or a similar spell cast at 5th Level or higher.
4- The target is Stunned for 1 minute. It may repeat the saving throw at the end of each of its turns, ending the effect on a success.
5- The target is teleported up to 120 feet to a location of Asharderis' choice. Asharderis may not choose a location closer to him than the target already was.
6- The target's maximum hit points are halved for one hour, or until the effect is dispelled by Remove Curse, Greater Restoration, or similar magic. This does not change the target's current hit points, although they're treated as temporary hit points while they're affected by this ability.
7- The target loses the ability to speak or write for 1 minute. 8- The target's weapon (if it has one) is torn from its hand and thrown 60 feet away from them in a direction of Asharderis' choosing.
9- The target is possessed by incorporeal demonic spirits. While possessed, the target is charmed, and views Asharderis as its lord and master, to be worshiped and protected. The target can repeat the saving throw at the end of each of its turns, ending the effect on a success. Otherwise, the possession lasts for 5 minutes.
10- The target gains a form of Indefinite Madness unique to Asharderis, and has permanent disadvantage on initiative rolls against demons.

Master's Call. If Asharderis isn't currently wielding the Staff of the Endless Abyss, as long as another creature hasn't attuned itself to the Staff, he can summon it back to his hand across any distance, including planar boundaries. If another creature has attuned itself to the Staff of the Endless Abyss, Asharderis instead receives a level of Exhaustion, regardless of his immunity to the condition.

Asharderis 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. Asharderis regains spent legendary actions at the start of his turn.

Attack. Asharderis makes one attack with his staff or uses Crimson Demonflame once.

Afflicting Grasp (Costs 2 Actions). Asharderis uses his Soul Shattering Touch.

Will of the Staff (Costs 2 Actions). The Staff of the Endless Abyss flares with unholy power. Each creature of Asharderis’ choice in a 20-foot radius must make a DC 24 Strength saving throw or take 27 (6d8) force damage, be pushed 10 feet away and be knocked prone. Asharderis can then teleport along with everything he's wearing or carrying up to an unoccupied space he can see within 60 feet of him.

Dominating Command (Costs 3 Actions). Asharderis attempts to seize control of a being's mind. One creature Asharderis can see within 60 feet of him must succeed on a DC 27 Charisma saving throw or become Charmed by the King of Demons. A creature that is Charmed in this way is under Asharderis' control during their next turn.

If Asharderis' Infernal Transcendence trait has activated in the last hour, he can use the options below as legendary actions.

Vent Rend. Asharderis causes the ground within 150 feet of him to split in a 30-foot line that is 5 feet wide. Each creature in that line must make a DC 25 Dexterity saving throw, taking 45 (13d6) fire damage on a failed save, or half as much on a successful one.

Cast a Spell (Costs 2 Actions). Asharderis casts a spell of 7th level or lower from his list of prepared spells, expending a use as normal.

To me (Costs 3 Actions). Asharderis begins concentrating for the next 2d2 turns. If this concentration isn't interrupted in some way, he conjures a Demon of CR 15 or lower in a location he can see within 120 feet of him.