The short-lived emperor Macrinus had a son whose name, inscriptions reveal, was M. Opellius Antoninus Diadumenianus. Little is known about this figure, who is remembered through brief references in the late Roman breviaries and in Herodian, and in a short biography in the collection of imperial lives now known as the Historia Augusta (= HA). In 1889, Dessau argued that the lives of the Historia Augusta, which present themselves as written by six different authors in the Age of Constantine, were in fact written by a single writer closer to the year 400, an argument that has now all but prevailed in scholarly circles. Some of the biographies depend on reliable sources, at least in part, and thus can provide actual historical information; others do not, and are mostly or entirely the result of authorial invention. The life of Diadumenianus fits clearly in the latter category. The secondary lives (Nebenviten) of the Historia Augusta contain no original information, but rather are constructed from a combination of information derived from their ‘parent’ life and invented fiction. So, for example, the life of the Caesar Aelius combines information from the life of Hadrian with fiction, the life of the usurper Avidius Cassius combines information from the life of Marcus Aurelius with fiction, and the lives of Pescennius Niger and Clodius Albinus combine information from the life of Severus with fiction. In the case of Diadumenianus, the life is even more thoroughly fictional, since the life of Macrinus from which it is derived is itself a kind of secondary life; Cameron suggests that Marius Maximus, the probable source for the author of the Historia Augusta, treated Macrinus as a usurper in the life of Elagabalus. Almost every detail of the life of Diadumenianus is therefore the fictive invention of the author.
