Archai

Devil's Sight. Magical darkness doesn't impede the devil's darkvision.

Magic Resistance. The devil has advantage on saving throws against spells and other magical effects.

PHASE 1:

Animorphus Resistance. The quick silver pupal form is absorbing power before the Archai's true form is revealed. The Archai has resistance to all physical damage in this phase.

Path To Hell. The Archai spawns 6 living embers at the beginning of any turn where there are no Living Embers on the battlefield and a turn that it did not use Explosive Apotheosis.

Explosive Apotheosis. This ability triggers at the beginning of its turn if Archai has 6 Motes of Ember. This ability deals 15 fire damage to every creature within 80 foot radius. The group may take a skill challenge with their reactions before the wave lashes out. If they succeed they will only take half damage.

PHASE 2: 280hp

Energy Overload. The chromatic orbs fuel the Archai with alien power to open a rip in reality to allow it to grip its weapon from a pocket dimension. The Archai has resistance to all damage while the orbs are active.

Energy Deflection. If all 4 of the chromatic orbs are covered or altered in a particular way to divert the arc of energy the Archai will drop to the ground until the beginning of the chromatic orbs next turn. During this time it loses Energy Overload.

PHASE 3: 160hp

Amber Resistance. The Archai can use its bonus action to change its resistance to all melee attacks or all range attacks. While resistant to melee attacks, the Archais form ripples with a subtle red glow. While the Archai is resistant to range attacks, the Archais form ripples with a subtle blue light.

Multiattack. The Archai makes three melee attacks: two with its brutal glaive and one with its tail. It can use Hurl Flame in place of any melee attack

PHASE 1:
Hurl Flame. Ranged Spell Attack: +8 to hit, range 150 ft., one target. Hit: 20 (5d6) fire damage. If the target is a flammable object that isn't being worn or carried, it also catches fire.

PHASE 2: 280

Hurl Flame. Ranged Spell Attack: +8 to hit, range 150 ft., one target. Hit: 20 (5d6) fire damage. If the target is a flammable object that isn't being worn or carried, it also catches fire.

Tail. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 10 (1d8 + 6) piercing damage. If the target is a creature other than an undead or a construct, it must succeed on a DC 17 Constitution saving throw or lose 10 (3d6) hit points at the start of each of its turns due to an infernal wound. Each time the devil hits the wounded target with this attack, the damage dealt by the wound increases by 10 (3d6). Any creature can take an action to stanch the wound with a successful DC 12 Wisdom (Medicine) check. The wound also closes if the target receives magical healing.

PHASE 3: 160

Brutal Glaive. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 15 (2d8 + 6) piercing damage.

Tail. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 10 (1d8 + 6) piercing damage. If the target is a creature other than an undead or a construct, it must succeed on a DC 17 Constitution saving throw or lose 10 (3d6) hit points at the start of each of its turns due to an infernal wound. Each time the devil hits the wounded target with this attack, the damage dealt by the wound increases by 10 (3d6). Any creature can take an action to stanch the wound with a successful DC 12 Wisdom (Medicine) check. The wound also closes if the target receives magical healing.

Hurl Flame. Ranged Spell Attack: +8 to hit, range 150 ft., one target. Hit: 14 (5d6) fire damage. If the target is a flammable object that isn't being worn or carried, it also catches fire.

Flame Wreath. As a bonus action the Archai creates a red glyph beneath 3 random targets. If the target does anything other than verbal commands the glyph erupts with magical energy in a 20-foot-radius sphere centered on the glyph. The sphere spreads around corners. Each creature in the area must make a Dexterity saving throw(The target is denied the save). A creature takes 5d8 fire damage on a failed saving throw or half as much damage on a successful one. 

Erupting Flame. The Archai arcs its blade and creates an eruption of fire and brimstone on the ground that shoots upward. You can make the flame up to 60 feet long, 20 feet high, and 1 foot thick. When the flame appears, each creature within its area must make a Dexterity 17 saving throw. On a failed save, a creature takes 5d8 fire damage, or half as much damage on a successful save. Erupting flame deals 5d8 fire damage to each creature that ends its turn within 5 feet of the outside of the flame or half as much on a successful dexterity save.