Valence

Valence is a Virus Entity born from the magical energies of the Millenium Comet, being harnessed in an attempt to summon a Groudon. Being a creature formed from poorly-harnessed magic and little else, Valence's "true" form consists of little more than a pile of teal slime, with flecks of glowing, light blue material flowing through it. They can, however, take on the form of any Pokemon they encounter and manage to get even the slightest bit of life-force from. As they wander aimlessly around the world, their collection of available forms grows ever larger as their unstable body saps the life-force from everything around them.

Under normal circumstances they will not enter a confrontation deliberately and will simply exist as a background entity shifting forms frequently, but if they are threatened, they can use the slimy tentacles sprouted from their back to wrap around and degenerate the bodies of their opponents, locking their souls inside Valence's body for as long as they continue to exist. It is believed that only the almighty Doom Desire of a Jirachi can neutralize the Millenium Comet magic holding them together, destroying them for good and returning their collection of captured souls to existence.

They have the mental capacity of an animal in their true form, and take on the personality and intelligence of whatever being they currently are taking the form of. Their "default" form mimics the personality and appearance of a Groudon (though no one's entirely sure which one), as that's what they were technically "supposed" to be.