Viridian, Archfiend of Waterdeep

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

Infernal Reconstruction (Recharges after a Short or Long Rest). If Viridian would be reduced to 0 hit points, her current hit point total instead resets to 450 hit points, she becomes a fiend, she loses the effects of The Devil's Mirror and the Pink Dress of the Archmagi, and she regains any expended uses of Legendary Resistance. Additionally, Viridian can now use the options in the “Mythic Actions” section for 1 hour. Award a party an additional 120,000 XP (240,000 XP total) for defeating Viridian after her Infernal Reconstruction activates.

The Devil's Mirror. This +2 shield gives Viridian resistance to Radiant and Necrotic damage.

Pink Robe of the Archmagi. Whilst wearing this silk dress, Viridian has advantage on saves to resist spells or other magical effects.

Multiattack. Viridian makes five attacks, with either Eldritch Blast or the Blade of Malbolge.

Eldritch Blast. Ranged Spell Attack: +20 to hit, range 120ft., one target. Hit. 16 (1d10+7) force damage on a hit.

Blade of Malbolge. Melee Weapon Attack: +20 to hit, reach 5ft., one target. Hit. 11 (1d4+9) slashing damage plus 10 (2d8) necrotic damage. A creature hit by the Blade of Malbolge loses maximum hit points equal to the necrotic damage dealt.

Infernal Spellcasting. Viridian can cast the following spells as part of her infernal heritage, using Charisma as the spellcasting ability (spell save DC 26):

At Will: Shield, Armor of Agathys, Inflict Wounds, Misty Step.

3/day each: Counterspell, Fireball, Raulothim's Psychic Lance, Hold Monster.

1/day each: Disintegrate, Time Stop, True Polymorph.

1 time: Wish.

Silver Tongue.  As a bonus action, Viridian can cast the Dominate Monster (Spell Save DC 26) spell on a humanoid within range that can hear her. If a creature passes the save to resist, it is immune to Silver Tongue's effects for the next 24 hours.

Fiery Rebuke (3/Day). When Viridian is damaged by a creature that she can see within 60 feet of herself, she magically engulfs the creature in flames. The creature must make a DC 26 Dexterity saving throw, taking 16 (3d10) fire damage on failed save, or half as much damage on a successful one.

Archfiend's Curse (1/Day). When Viridian is damaged by a creature that she can see within 60 feet of herself, she can choose to hex the creature. Whilst hexed, Viridian scores a critical hit on a 19 or 20 on a D20 attack roll against the afflicted creature, and she deals an additional 8 damage of the weapon or spell's type.

Viridian 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. Viridian regains spent legendary actions at the start of its turn.

Attack. Viridian makes two attacks with Eldritch Blast or the Blade of Malbolge

Spellcasting (Costs 2 Actions). Viridian casts a spell from the Infernal Spellcasting spell list.

If Viridian's Infernal Reconstruction trait has activated in the last hour, it can use the options below as legendary actions.

Eldritch Beam. Viridian fires all five Eldritch Blast  charges at one target. If hit by one or more beams, the target must succeed on a DC 26 Strength saving throw or be knocked prone at the end of the attack sequence.

Infernal Transformation (Costs 3 Actions). Viridian targets one object within range for the True Polymorph spell, transforming it into a Pit Fiend (Monster Manual).  This Pit Fiend is hostle to players and allied with Viridian.