Outpost 13 : Desolace Series III
by Lucian Barnes
As Edward and his makeshift group of adventurers descend the stairs leading underground from the Outpost, they find a strange object as well as Katie's friend, Julie. They watch in horror as Melissa disappears through this new portal, and decide to follow her. When they get to the other side, they find themselves in a post-apocalyptic world that resembles Earth.