Not long after this, Constantine the younger, who was the brother of Emperor Constantius and was named after their father, invaded the territory of his younger brother Constans and engaged with his soldiers. He was killed by them in the consulship of Akindynos and Proklos [340 C.E.].
Μετ΄ οὐ πολὺ δὲ ὁ τοῦ βασιλέως Κωνσταντίου ἀδελφὸς ὃς ἦν ὁμώνυμος τῷ πατρὶ͵ ὁ νέος Κωνσταντῖνος͵ ἐπιὼν τοῖς μέρεσι τοῦ νέου ἀδελφοῦ Κώνσταντος͵ συμβαλών τε τοῖς στρατιώταις αὐτοῦ͵ ἀναιρεῖται ὑπ΄ αὐτῶν ἐν ὑπατείᾳ Ἀκινδύνου καὶ Πρόκλου.
—Socrates Scholasticus, Historia ecclesiastica 2.5
Interesting to specify the soldiers in particular; Constantine died in a minor action outside Aquileia, not a pitched battle between armies, and his body was thrown into the nearby Alsa River.