Flavius Julius Constans (c. 323 – 350), also called Constans I, was Roman emperor from 337 to 350. He held the imperial rank of Caesar from 333, and was the youngest son of Constantine the Great.
After his father's death, he was made Augustus alongside his brothers in September 337. Constans was given the administration of the praetorian prefectures of Italy, Illyricum, and Africa. He defeated the Sarmatians in a campaign shortly afterwards.[4] Quarrels over the sharing of power led to a civil war with his eldest brother and co-emperor Constantine II, who invaded Italy in 340 and was killed in battle by Constans's forces near Aquileia. Constans gained from him the praetorian prefecture of Gaul. Thereafter were tensions with his remaining brother and co-Augustus Constantius II (r. 337–361), including over the exiled bishop Athanasius of Alexandria,[4] who in turn eulogized Constans as "the most pious Augustus... of blessed and everlasting memory.  "In the following  years he campaigned against the Franks, and in 343 he visited Roman Britain, the last legitimate emperor to do so   until Manuel II in 1400, more than a thousand years later.
On 18 January 350,[38] the general Magnentius declared himself emperor at Augustodunum (Autun) with the support of a number of court officials such as Marcellinus, Constans' comes rerum privatarum, as well as Fabius Titianus, who had previously served as the praetorian prefect of Gaul. At the time, Constans was distracted by a hunting trip.[39] As he was trying to reach Hispania, supporters of Magnentius cornered him in a fortification in Helena (Elne) in the eastern Pyrenees of southwestern Gaul, where he was killed after seeking sanctuary in a temple. An alleged prophecy at his birth had said Constans would die "in the arms of his grandmother". His place of death happens to have been named after Helena, mother of Constantine and his own grandmother, thus realizing the prophecy. Constans' name would later be erased from inscriptions in places that recognized Magnentius as emperor.[