A transdimensional being, it could travel through time and space at will. The Seventh Doctor described it as an elemental that, though essentially spiritual, could interact with the physical world. When assuming human form, it appeared as a tall, thin, elegant man with a long thin aristocratic face. It influenced Earth history, increasing the level of conflict during the French Revolution and the First World War, as well as a number of other events.
The Committee of Three tricked the Doctor into tracking down Agonal so they could capture it. They used a Time Scoop to bring it to Gallifrey. Since it drew strength from its opponent, they planned to use it to fight and destroy Rassilon.
First, using a Time Gun, they freed Borusa from his imprisonment in Rassilon's tomb, so he could lead them. However, Borusa rejected them, having contemplated his past actions during the time in the tomb and repented. He joined minds with the Doctor and Romana II to help Rassilon defeat Agonal, who put him in the same place where Borusa had been trapped. (PROSE: Blood Harvest)