Gallienus was challenged by Aureolus

In 268, at some time before or soon after the battle of Naissus, the authority of Gallienus was challenged by Aureolus, commander of the cavalry stationed in Mediolanum (Milan), who was supposed to keep an eye on Postumus. Instead, he acted as deputy to Postumus until the very last days of his revolt, when he seems to have claimed the throne for himself. The decisive battle took place at what is now Pontirolo Nuovo near Milan; Aureolus was clearly defeated and driven back to Milan. Gallienus laid siege to the city but was murdered during the siege. There are differing accounts of the murder, but the sources agree that most of Gallienus' officials wanted him dead.