Eternal War
From Tardis Wiki, the free Doctor Who reference
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 which saw star systems destroyed and billions killed though ultimately Gallifrey was victorious in the conflict with the creatures expelled from the universe. It was the Gallifreyan General Kopyion Liall a Mahajetsu who led the Time Lords against the Yssgaroth and during that time Rassilon rose 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 and its details were never recorded in the Matrix. (NA: The Pit)
