Gallienus was a Roman Emperor who ruled from 253 to 268 AD. He was the son of Emperor Valerian and became co-emperor with him. After his father's capture by the Persians, Gallienus faced numerous challenges, including invasions and internal strife within the Roman Empire. During his reign, Gallienus is known for his military reforms and efforts to stabilize the empire. He promoted the use of cavalry and improved the organization of the army. Despite facing significant threats, he managed to maintain control over much of the empire until his assassination in 268 AD.