The Ecclesiastical History of Sozomen

Four years after the council of Sardica, Constans was killed in Western Gaul. Magnentius, who had plotted his murder, reduced the entire government of Constans under his own sway. In the meantime Vetranio was proclaimed emperor at Sirmium, by the Illyrian troops. Nepotian, the son of the late emperor’s sister, gathered about him a body of gladiators, and wrangled for the imperial power, and ancient Rome had the greatest share of these evils. Nepotian, however, was put to death by the soldiers of Magnentius. Constantius, finding himself the sole master of the empire, was proclaimed sole ruler, and hastened to depose the tyrants. In the meantime, Athanasius, having arrived in Alexandria, prepared to convene a Synod of the Egyptian bishops, and had the enactments confirmed which had been passed at Sardica, and in Palestine, in his favor.