“I tell ye this and I charge ye with my words: In ancient times, a rebel spirit of violence and chaos rampaged across the world, laying all it saw to waste, leaving a path of destruction in its wake. Fearing for its creation, the Original One cast the being away across the threshold of dimension - but to teach it the value of preservation, he gave it a world to rule alone, unchallenged. Where there was one world there were now two, back to back - only visible to each other where the fabric of reality is weakened - but strongly bound together.
Lo - the beast that sought to unmake the world, became its guardian - its role reversed.”
Giratina, the Renegade, In a moment between formes, between worlds.
Giratina: Between Worlds
by StellarWind Elsydeon