Updated with the basic plot of the story.
A long battle raged across Unova destroying anything that happened to be in its path. Two sides were in this battle: On one side there were Reshiram and Zekrom and on the other side was their long time enemy Kyurem. Though the Battle was long and destructive in the end Kyurem retreated and hasn't been heard from since, however Zekrom had been severely weakened during the battle that even being a Legendary couldn't save him. Reshiram was distraught over this and did everything he could to save his brother, but to no avail. In a last ditch effort to keep himself alive and preserve the balance in the world he placed the last of his energy within a stone that only his true successor could open and release. Reshiram didn't like this but seeing as how he would do anything to see his brother alive and well again went along with it. Reshiram then decided he would stay in the Dragonspiral Tower until the day when his brother could