Some of Numerian's soldiers smell an odor reminiscent of a decaying corpse emanating from his coach after the army has traveled through Bithynia to reached the Bosporus late in 284.
They open its curtains and inside find Numerian, dead.
His father-in-law and adoptive father, the praetorian prefect Flavius Aper, assumes command but is accused having killed the emperor in order to seize power.
Instead, Diocles, who may have been a member of Carinus' bodyguard, is acclaimed as emperor by his fellow soldiers.
The thirty-nine-year old army chief, whose father had been a scribe or the emancipated slave of a senator called Anullinus, has risen from modest Dalmatian origins through the military ranks to become Dux Moesiae, with responsibility for defending the lower Danube.
Appearing for the first time in public dressed in the imperial purple, he declares himself innocent of Numerian's murder, designates Aper as the criminal and kills him personally.
Contemporaries accept Aper's guilt, but it is also the case that a prediction had been made to Diocletian previously, telling him that he would become emperor on the day he killed a boar (Latin: aper).
By eliminating Aper, Diocletian has rid himself of an eventual competitor and, retroactively, provided his act with sacred meaning.
Acclaimed emperor on November 17, 284, Diocletian possesses real power only in those countries that are dominated by his army (i.e., in Asia Minor and possibly Syria).
The rest of the empire is obedient to Numerian's brother Carinus.
