Dark Matter

Supposedly she is an otherworldly being that passed onto the Pokemon world, where she possessed a female Umbreon, using their body to drain away darkness from any living Pokemon in her way, all from a thirst for power. Unfortunately this host met her end as Cerberus too began to absorb dark magic in desperation of restoring his life back to before his father met his demise, or at most replicate it. But he didn't realize the only reason this one was so powerful, was because of Dark Matter, who he absorbed by mistake, and inviting her right into his consciousness. He attempted to reason with Dark Matter that her power lust is getting her nowhere, and convert her to good, but it fell upon deaf ears, and Cerberus, having allowed her to gain so much power, was unable to resist her influence, and was easily overthrown of control.

No more Cerberus. Only Dark Matter.

Now that it was her body, she modified it to her own image, now she is known as the most powerful dark sorceress in the Pokemon universe, forever seeking more and more power...