The Eternal War was a devastating conflict between the Time Lords and the extradimensional beings known as the Yssgaroth.
The war between the two lasted for a thousand years, and saw star systems destroyed and billions killed. Ultimately, Gallifrey was victorious, and their foes were expelled from the universe. The Gallifreyan General Kopyion Liall a Mahajetsu led the Time Lords against the Yssgaroth. During that time, Rassilon began his rise to power. After the Eternal War, Kopyion and two surviving soldiers discovered that their world's religion had been abolished in favour of science. In time, the memory of the war became legend. However, its details were never recorded in the Matrix. (PROSE: The Pit)