Answer
Magnentius, a general in the Roman army, overthrew and killed Constans, the Roman emperor, in 350 AD. Magnentius had been passed over for promotion in favor of Constans' brother, Constantius II, and he used this as an excuse to rebel. He gathered a large army and marched on Constans, who was defeated and killed in the battle of Mursa Major. Magnentius then declared himself emperor, but his reign was short-lived.