Nightfall (Stargate Atlantis #10)

In the distant Pegasus galaxy, the scientists and soldiers of the Atlantis expedition push back the frontiers of knowledge and exploration; in the city of the Ancients, new challenges and old secrets are waiting to be unlocked. Far from home, ranged against lethal enemies the Atlantis team must fight to preserve a distant outpost of humanity against impossible odds; but when the Nightfall comes, it will be like nothing they have ever encountered before...