Gallienus (22 October 253 - September 268CE). Publius Licinius Egnatius Gallienus Augustus became co-emperor with his father, Valerian, in 253. He became sole emperor upon Valerian's capture by the Persians in 259/260. Although an able military leader, he was unable to stem the disintegration of the Empire. He was assassinated in a conspiracy involving Illyrian officers and the future emperors Claudius and Aurelian at the Siege of Milan in September 268.