The Ecclesiastical History Of Scholasticus by Socrates Scholasticus
Chapter XXIV.--Valentinian a Son of Constantius and Placidia, Aunt of Theodosius, is proclaimed Emperor.
After the usurper's death, the emperor Theodosius became very anxious as to whom he should proclaim emperor of the West. He had a cousin then very young named Valentinian; the son of his aunt Placidia, daughter of Theodosius the Great, and sister of the two Augusti Arcadius and Honorius and of that Constantius who had been proclaimed emperor by Honorius, and had died after a short reign with him. This cousin he created Cæsar, and sent into the Western parts, committing the administration of affairs to his mother Placidia. He himself also hastened towards Italy, that he might in person both proclaim his cousin emperor, and also being present among them, endeavor to influence the natives and residents by his counsels not to submit to usurpers readily. But when he reached Thessalonica he was prevented from proceeding further by sickness; he therefore sent forward the imperial crown to his cousin by Helion the patrician, and he himself returned to Constantinople. But concerning these matters I deem the narrative here given sufficient.