Pokémon: The legend of the Crystal Moon
When a Pokémon is treated in a very cruel and evil way it will close its heart. If a Pokémon ever closes its heart it will awaken the spirit of the legendary shadow trainer. He was a very cruel trainer who used brute force and pain to make his Pokémon strong. He was so powerful not a trainer in the world could beat him. He would steal other trainers Pokémon that he wanted and raise them in this way so it would become powerful but the Pokémon would never be the same. Only he had complete control over shadow Pokémon. Long ago he finally passed away and it’s said he spirit was sealed away with the spell, Pokémon tears and a promise that the Pokémon will never be harmed or treated in an evil way again. All Pokémon he corrupted were eventually purified. Most of them could not be saved and had to be locked away. Some still remain under the care of a scientist who takes care and studies the Pokémon. If someone does manage to close a Pokémon’s heart his spirit will rise and corrupt that trainer. Once he rises he will block out the sun drowning the world in darkness so he may eventually corrupt all the Pokémon in the world and rule forever.
NOTE: The Shadow doesn’t affect the stronger trainers, Such as GYM leaders among a few other trainers.