Bessan was a planet home to the species of the same name.
As Brother Lassar disclosed whilst speaking with the Tenth Doctor, the people of Bessan had "some rather lovely wings". Before the year 1140, and ten generations prior to 2007, Bessan was invaded by the Krillitanes, with Lassar recalling that Bessan made a million widows in one day. The Krillitanes adapted the wings of the Bessan people to assume a bat form for their newer generations. The Esteemed Father, of the older, wingless generation, was recognised as the "Beast of Bessan". Following his encounter with Lassar from his perspective, the Doctor met Krillitanes who he surmised were of the first generations to have adopted the wings of the Bessan. He was aware that the entire Bessan civilisation was slaughtered. (TV: School Reunion, PROSE: The Krillitane Storm)