Dual Essence (Mythic). Zevalch is composed of two converging halves - corrupted and divine. At the start of each of its turns, it switches to the opposite form. 

Corrupted Form Its melee attacks deal an additional 1d8 necrotic damage, and enemies within 10 ft. have disadvantage on Constitution saving throws. 

Divine Form It gains resistance to all damage until the start of its next turn and heals 10 HP. 

Multiattack. Zevalch makes three melee attacks: one with each Claw and one with either Crescent Rend or Disjointed Beam.

Claw. Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 13 (2d8 + 4) [slashing] damage.

Crescent Rend. Melee Weapon Attack: +9 to hit, reach 5ft., one target. Hit: 15 (2d10+4) [slashing] damage. The target must make a DC 16 Constitution saving throw or lose concentration on any spell, and take an additional 2d6 psychic damage. 

Disjointed Beam (Recharge 5-6). Ranged Weapon Attack: +10 to hit, range 90 ft., one target. Hit: 27 (6d8) [force] damage. On a failed DC 16 Strength save, the target is pulled 15 ft. toward Zevalch. 

Catalyst Feedback (1/Turn). When Zevalch is targeted by a spell of 3rd level or lower, it can use its reaction to force the caster to make a DC 16 Constitution saving throw.
On a fail, the caster takes 2d8 psychic damage and must roll on the Wild Magic Surge table. (You can flavor this as a burst of unstable catalyst energy reacting to arcane influence.)

 Flicker Step (Cost 1 Action). Zevalch teleports up to 40 ft. to an unoccupied space it can see. 

Black Hole Pulse (Cost 2 Actions). Zevalch targets a 20-ft-radius sphere centered on itself. All enemies must make a DC 17 Constitution saving throw. On a fail, they are stunned until the end of their next turn. On a success, they are pushed 20 feet away and take 3d10 force damage.

Surge of Divinity (Costs 3 Actions). Zevalch regains 20 hit points and emits a radiant shockwave. All enemies within 15 ft. must make a DC 17 Constitution saving throw or be blinded until the end of their next turn. 

 Severance Rite (Mythic Surge, 1/Encounter). When Zevalch is reduced to 0 HP for the first time, it instead remains at 1 HP and sheds its physical form in a burst of magical energy. All creatures within 60 ft. must succeed on a DC 16 Dexterity saving throw or take 6d10 force damage and be knocked prone. Zevalch then regains 100 HP, and all conditions affecting it are removed. The battlefield trembles, and its wings grow with arcane black flame.