Orosius, History Against the Pagans 7.33:
“Meanwhile in parts of Africa, Firmus made himself the king of the provoked tribes of the Moors and laid waste to Africa and Mauretania. He gave Caesarea, the noblest city of Mauretania which had been taken by fraud and then filled with murder and flames, to the barbarians as a prize. Then count Theodosius, the father of the Theodosius who later ruled as emperor, was sent by Valentinian and broke the tribes of the Moors, who were scattered after many battles. He drove Firmus, now afflicted and oppressed, to death. Afterward, when he had with the most knowing foresight returned all of Africa with Mauretania to their previous states, he received the order (thanks to provoking and clamoring envy) that he was to be executed. He chose to be baptized in Carthage in order to remit his sins, and after he attained the sacrament of Christ which he had sought, he cheerfully offered his neck to the executioner following a glorious life, and secure of the life hereafter.”
Interea in Africae partibus Firmus sese excitatis Maurorum gentibus regem constituens Africam Mauretaniamque uastauit; Caesaream urbem nobilissimam Mauretaniae dolo captam, deinde caedibus incendiisque conpletam barbaris in praedam dedit. igitur comes Theodosius, Theodosii qui post imperio praefuit pater, a Valentiniano missus effusas Maurorum gentes multis proeliis fregit, ipsum Firmum afflictum et oppressum coegit ad mortem. post cum experientissima prouidentia totam cum Mauretania Africam meliorem pristinis reddidisset, instimulante et obrepente inuidia iussus interfici, apud Carthaginem baptizari in remissionem peccatorum praeoptauit ac postquam sacramentum Christi quod quaesierat adsecutus est, post gloriosam saeculi uitam etiam de uitae aeternitate securus percussori iugulum ultro praebuit.