The Grey Wizard and The Dragon Rider

Soon after leaving Bilbo Baggins at the border of the Shire, Gandalf the Grey is transported to another world. There he meets Eragon, Murtagh, and Saphira, on their quest to the Varden. Gandalf joins them, hoping to find some way back home. How will the story go now? And, who sent him here? Was this the Valar, or the machinations of Sauron?