Every couple thousands of years evil gathers into a single source. This source is the embodiment of evil, hatred and everything evil.
It takes on the form of whatever it can. This all depends on it's power.
Less power means no defined form, so it is less dangerous. This was the Bittercold two thousand years ago.
The form also depends on where it is gathered. However, if hatred and evil in the world grows to an unimaginable strength, it gains a form of a Pokémon.
What Pokémon that is shows little information.
The only answer gotten from what Pokémon it is would seem to be the evil having a lot of strength. This is false, as the evil is unbeatable in this form.
Only divine intervention could save the world then, unless a different miracle happens. The evil is beatable by the kindest and purest of hearts.
The days when that could happen had finished millennia ago. Everyone knows the evil is forming, and they know only a human could take it out.
The humans had all died by this time. With feelings of hope gone, and hopeless feelings in their heart, they know they had to put their faith into the divine.