Two years ago, Reshiram and the hero N fought with Zekrom and the hero whom it had partnered with. The two put their beliefs on the line and dueled for the future of Pokémon and humanity.
N and Reshiram were victorious.
Pokémon were seperated from humans, shipped overseas to undiscovered regions. Poké Balls were outlawed and Team Plasma, the disciples of the hero N, enforced this New World Order.
But N's father, Ghetsis, betrayed him. Ghetsis took another dragon, Kyurem, and fused it with Reshiram, creating White Kyurem, ruling with an iron fist. Resistance was useless, for even the PC network was hacked and the Pokémon inside released. Team Plasma's rule was spread worldwide over time, and eventually the entire planet was under the dictation of Ghetsis.
Zekrom and the two heroes vanished, fleeing, but they were soon caught. The heroes were executed and Zekrom was returned to its dormant state, the Dark Stone, and hidden away.
But in secret, a guild was formed between the few remaining Trainers left. They are the world's final hope. And so are you.
Welcome to the Last Alliance.
Oh, and if you prefer to have Ghetsis use Black Kyurem, see this RP's counterpart, Black Plasma.