Cassius Eldon was an African-American who helped to run the slave railroad in and around Richmond, Virginia at around the time of the American Civil War. He met the Fifth Doctor when the Doctor was looking for Peri and Erimem. His status as the informal leader of the railroad gave him deep access to news in the Richmond area, and thus he was well-placed to help the Doctor find his lost companions.
As a leader in the movement to free slaves, he of course knew CSA Colonel Jubal Eustace, whom he considered "crazy evil". He went on to explain, "Eustace likes to kill us coloured folks. Eustace likes to kill any kind of folks. This war gave him the chance to kill as many as he cared to. He's a man I don't ever care to meet." (PROSE: Blood and Hope)