The Ecclesiastical History of Socrates Scholasticus

Under the consulate of Gratian and Dagalaïfus in the following year, the war was begun. For as soon as the usurper Procopius, leaving Constantinople, began his march at the head of his army toward the emperor, Valens hastened from Antioch, and came to an engagement with him near a city of Phrygia, called Nacolia. In the first encounter he was defeated; but soon after he took Procopius alive, through the treachery of Agilo and Gomarius, two of his generals, whom he subjected to the most extraordinary punishments. The traitors he caused to be executed by being sawn asunder, disregarding the oaths he had sworn to them. Two trees standing near each other being forcibly bowed down, one of the usurper’s legs was fastened to each of them, after which the trees being suddenly permitted to recover their erect position, by their rise rent the tyrant into two parts; and thus torn apart the usurper perished.