An individual claiming to be related to the Emperor Romanos IV Diogenes (1068–71) took part in a major Cuman invasion of Byzantium on the eve of the First Crusade. This article assesses the date of the assault, which is recorded by the Russian Primary Chronicle, by John Zonaras, and by Anna Komnene in the Alexiad. The identity of the man referred to as the False Diogenes by the Byzantine sources is considered, and it is argued that, rather than being an impostor, the individual in question may well indeed have been the son of Romanos IV Diogenes. Modern scholars have tended to ignore this possibility, instead following Anna Komnene’s meticulous character assassination of the man who accompanied the nomad attack. This paper therefore also seeks to address the question of the identity of ‘Pseudo-Diogenes’, to examine Anna Komnene’s methods of savaging a natural rival to her father for the imperial throne, and to assess her motives for doing so.