The Krillitanes invaded Bessan some time before the year 1140 and ten generations prior to 2007.
Brother Lassar recalled that the Bessan people made a million widows in one day. The Krillitanes adapted the "rather lovely 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 Tenth 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)