Once, long, long ago, there were three powerful legends. Dialga, Palkia, and Giratina.
Each one had their own Mythical, that they looked after like a child. Celebi for Dialga, Hoopa for Palkia, and Marshadow for Giratina.
While Dialga and Giratina often got along with their Mythicals, Palkia did not. Hoopa was a great prankster, always causing trouble.
One day, Hoopa decided that they would try to make their own landmass. And so, they coerced Groudon to start raising some land from the sea.
However, Kyogre caught wind of this, and a great battle was fought between the legendaries of land and sea. Many Pokemon died.
When Arceus caught wind of this, they demanded to know who had caused this, who had coerced Groudon. And Hoopa, the trickster, had managed to convince everyone that it was Palkia who had done it.
However, Giratina did not wish for their sibling to be punished. They said they were the one who coerced Groudon.
And so, as punishment, Arceus banished Giratina to the Distortion World.