Ammianus Marcellinus Res Gestae val1.4

[9] Severus Caesar was low both in character and in origin, given to drink, and hence a friend to Galerius. Accordingly Galerius made Caesars of him and Maximinus, without Constantine having knowledge of any such step. To this Severus were assigned some cities of Pannonia, Italy, and Africa. Through this chance Maxentius became emperor; for Severus was deserted by his men and fled to Ravenna.

[10] Summoned to support his son Maxentius, Herculius came to Ravenna, deceived Severus by a false oath, gave him into custody, and took him to Rome in the condition of a captive; there he had him kept under guard in a villa belonging to the state, situated thirty miles from Rome on the Appian Road. ¹ When Galerius later went to Italy, Severus was executed; then his body was taken to a place eight miles from the city, and laid in the tomb of Gallienus.

[11] Now Galerius was such a tippler ² that when he was drunk he gave orders such as ought not to be obeyed; and so, at the advice of his prefect, he directed that no one should execute any commands which he issued after luncheon.

[12] Meanwhile Constantine, after defeating the tyrant’s ³ generals at Verona, went on to Rome. When he had reached the city, Maxentius came out and chose a plain ⁴ above the Tiber as the place to do battle. There the usurper was defeated, and when all his men were put to flight, he was prevented from escaping by the crowd of fugitives, thrown from his horse into the river, and drowned. On the following day his body was recovered from the Tiber, and the head was cut off and taken to Rome. When his mother was questioned about his parentage, she admitted that he was the son of a Syrian. He ruled for six years. ⁵