Tenebrae

Tenebrae was known as the Demon Pokémon; he took the form of a giant ninetales composed completely of black tar-like slime. He was the amalgamation of three pokémon gods; two ninetales and an arcanine. One by one, these three pokémon gods broke ancient laws and meddled with the world, and after a while of this they disappeared and formed Tenebrae. It was after the three pokémon gods disappeared that Mew had her prophecy about Stun.

Tenebrae was the source of all the darkness, and wanted to rule the planet with an army of possessed pokémon. It was revealed in an alternate timeline that if Stun had not been there to stop him, the darkness would eventually control all life and result in the planet imploding.

Tenebrae was first introduced in Chapter 15 of Stun and was defeated and killed by Stun in Chapter 25, resulting in the three pokémon gods being released and whole once again.

Tenebrae makes another appearance in Chapter 15 of Stun II when Stun goes back in time in an attempt to save Star.

Trivia:
 * 1) Tenebrae's name was originally Vampiro, before the author decided that he wanted a more serious name for the Demon Pokémon.