The Battle of Mursa Major was fought in 351 between a Roman army led by Constantius II and the forces of the usurper Magnentius. The battle was one of the bloodiest in Roman history, but Constantius emerged victorious. Magnentius would suffer another defeat two years later at Mons Seleucus. Events March 15 - Constantius II elevates his cousin Gallus to Caesar, or assistant emperor, and is put in charge of the Western Roman Empire. ... emperor Constantius II Constantius II, Roman Emperor (7 August 317 - 3 November 361, reigned 337 - 361), was the middle of the three sons of Constantine I the Great and Fausta. ... Magnentius (ruled AD January 18, 350–August 11, 353), was a Roman usurper. ...