Gaius Aurelius Valerius Diocletianus, born in Dalmatia, overcame a humble background by virtue of his military prowess. He commanded the imperial bodyguards of the emperors Carus and Numerian, both of whom were nevertheless murdered. Diocletian scapegoated Arrius Aper, the praetorian prefect, for the latter's death, executed him, and was acclaimed emperor. Numerian's brother Carinus led an army against Diocletian's forces, but improbably was assassinated during the key battle by one of his subordinates. Thus it was that Diocletian survived this challenge to his legitimacy.
As emperor, Diocletian engaged in numerous campaigns against barbarians and usurpers alike, but he's most remembered for insituting a tetrarchy under which the empire was partitioned and each portion ruled by an Augustus with a Caesar beneath him whose sucession to the throne was assured upon the former's death. Diocletain also reformed the army, and greatly increased the number of provinces (thus reducing the power of any individual governor).
Diocletian adopted the airs of divinity, and attempted to restore traditional Roman religious worship. Not surprisingly, he persecuted Christians in trying to achieve this goal.
After two decades of rule, Diocletian abdicated for reasons of health, and died of natural causes a half dozen years later.


This specimen is classified as an AE 20, I have it as RIC VI 13b (Sear 12636). It was struck at Heraclea.