The Forgotten

The Forgotten were a group of Time Lords who worked on the neural mechanics of the Matrix in the ancient days of Gallifreyan history. They discovered that by converting themselves to aggressive electrochemical impulses, they could travel back in time through the mind of another Time Lord, following his personal time stream backwards. The Eighth Doctor encountered the Forgotten in an ancient TARDIS, and with the help of a future-self from an alternate timeline, trapped them inside a portion of his brain that would be destroyed when he next regenerated. (PROSE: Dead Time)