Hero of Truth

Reshiram was commanded by the Hero of Truth one thousand years prior to the events of the films. At the end of their war with the Hero of Ideals and his Zekrom, it returned to the form of a Dragon Stone and was sealed away in the caves inside the Sword of the Vale. During the actual events of the films, Reshiram helps Zekrom save the world from the corrupted Dragon Force by funneling it into the Sword of the Vale. It also aids by trying to destroy the Protecting Pillars and then by helping guide the Sword of the Vale back down to the ground.