Answer
Gallienus met his demise through an assassination plot orchestrated by his own officers, driven by their resentment towards his leadership and their desire for power. This treacherous act occurred in 268 CE, while Gallienus was actively engaged in a military campaign against the invading Goths. His untimely death plunged the Roman Empire into further chaos and instability, contributing to the tumultuous period known as the Crisis of the Third Century.