Urnud is a planet in a different cosmos in the Polycosm which has beings similar to humans. During a world war, they developed ships powered by detonating nuclear weapons against a blast plate. The largest of these ships, the Daban Urnud, was built for interstellar colonization.
An Urnud military fleet has a Gan (Admiral) and ships have a tactical Captain (Prag). The third Gan of the ship received a message he perceived as coming from the past of Urnud, but which may instead have been sent by Avout at the time of the Third Sack.
The Gan attempted to send the ship back in time to answer the summons, but instead it moved into the cosmos of Tro.