Ángel

Her anthem is
Worship her, no cacophony
The Fire sings her symphony
Progenitor of the troublesome three
In the end, she'll have all the glee.
She is the creator of Meatcanyon Lilly, Slasher Jr., and Petoster, the Troublesome Three.

Meatcanyon Lilly imgflip.com/i/axcjl3
Petoster imgflip.com/i/az2j41
Slasher Jr imgflip.com/i/axj8ti

She's a fallen angel that resembles Lilly. Her true form will be drawn in the future.

The reason for her becoming a fallen angel is because Arceus banished her not just to the distortion world, but to actual hell. She managed to destroy the original ruler of the underworld with just a flick of her leaf fingers. She killed a Torkoal and now wears its skull on her face which has now fused to her face. Overtime, she created 3 Guardians for herself. All tortured, her first creation had an error where she got infected by a cordyceps fungus. The other 2 came out perfectly though. One of them lies patiently until her demise, one stalks in the surface world waiting patiently for a moment to strike a kill, and finally, one is a master döppelgänger with the imposter title and is the only male of the troublesome three.

How she activates her true form is that she will absorb all 3 of her guardians and steal their powers. Her true form resembles a humanoid very far from looking like a pokemon. The face has entirely melted off replaced with a skull with a very gaping jaw. In her true form, she is the strongest thing in the universe, even overtaking Arceus.

You can also call her by a number of different names. The Dark Lady, Lilinot, Anti-Dark Lord, Face Snatcher etcetera.

The second I saw the word Diablo I thought “holy shit is this a Diablo 3 reference)

Sweet. 👍