This story is based off Pokemon Insurgence (fan-made game) so if you don't understand something in the story, it was probably in Pokemon Insurgence.
Prologue:
After Lord Arceus banned Giratina to the distortion world, all Giratina could think about was revenge. It has been controlling legendaries, and plotting a plan to destroy the Earth that Arceus has protected for generations. Giratina planned to kill every single living soul on the Earth with his gang of legendaries. Giratina was about to activate the ultimate plan to destroy everything Arceus loved and protected. He had collected the three crystal shards, one for Dialga, Palkia, and Giratina, however after Giratina was banished to the Distortion World, Arceus took Giratina's crystal, which had the potential to make any legendary turn Primal (with the exception of Kyogre and Groudon.) However, while Arceus has been asleep for the last thousand years, Giratina took all the crystal pieces, restoring him of his former power.
Oh, I should mention this. The evil teams have evolved into straight up cultists, and worship legendaries, trying to control them for their own selfish reasons. You will see how this affects the main characters later on. This should give you enough background information about the story, because I won't spoil any more of the plot. Anyway, remember to submit those forms, as I have many slots left. To be continued...