Yggdrasil The New Saga - Yggdrasil, #1

Jason and Cassandra discover they are the last heirs of Odin and must reclaim the throne of the Allfather to ensure the survival of the human race in a war against the Orcs coming from the outer lands of Middle Earth (Midgard). They also find out that Ragnarok already happened in the past, and the Old Gods remain in Valhalla while the dark creatures have taken over Asgard and the Nine Worlds ever since.