Just an Idea
Centuries ago in the early Pokémon world, wars covered it completely. Types fighting against each other, wanting to prove that they are the strongest. The wars were so catastrophic that not even Arceus could stop it. It looked like that the world was doomed to darkness.
Until one Pokémon stopped it all. The Pokémon looked that of an ice type, but it carried attributes of every type imaginable. The Pokémon discovered that darkness was taking control of Pokémon; clouding their judgment and filling their hearts with hate.
The hero figured a way to free them by using special crystals that represent every legendary Pokémon. By using the crystals, the hero brought peace and happiness to the land' finally the wars were over. All the Pokémon in the world rejoiced for peace.
However there was one Pokémon that didn't celebrate, in fact it was the Pokémon that started it all. Nuxis, no one knows what it looks like, all they say is that it takes form of a dark mist and takes over the victim of its choosing. Nuxis wanted a world plunged in chaotic darkness, so when it saw the hero free the others from his control all it wanted was revenged. So it bid its time plotting until it finally had the chance.
The two engaged in an epic battle neither side letting up. But the hero knew it could not last for long so it sealed Nuxis away in a magic stone, to be locked for centuries. But before Nuxis was sealed away he vowed that he would return and once again plague the world in darkness.
The Pokémon once again rejoiced that the darkness was sealed away, however, that joy was short lived when the hero collapsed. Knowing that Nuxis will return; Arceus gave the hero a new life, hoping that it will once again defeat the evil.
It wasn't until centuries later that the hero was once again born.