2. FLAVIA MAXIMA CONSTANTIA, the daughter of the emperor Constantius II. and his third wife, Faustina, was born shortly after the death of her father in A. D. 361. In 375 she was destined to marry the young emperor Gratian, but, on her way to the emperor, was surprised in Illyria by the Quadi, who had invaded the country, and would have been carried away into captivity but for the timely succour of Messalla, the governor of Illyria, who brought her safely to Siriniuum. When a child of four years, she had the misfortune to be seized with her mother by Procopius, a cousin of the emperor Julian, who had raised a rebellion in 365, and who carried his captives with him in all his expeditions, in order to excite his troops by their presence. Constantia died before her husband Gratian, that is, before 383, leaving no issue. (Amm. Marc. 21.15, 25.7, 9, 29.6.)
[W.P]A Dictionary of Greek and Roman biography and mythology
Smith, William
A Dictionary of Greek and Roman biography and mythology. William Smith, LLD, ed. 1890