Complex sentence: On the river Frigidus he defeated Arbogast in AD 394 and thereafter had Eugenius executed; however, in January AD 395 Theodosius himself died at Mediolanum (Milan).
Split into simpler sentences: On the river Frigidus he defeated Arbogast in AD 394 and thereafter had Eugenius executed. For already in January AD 395 Theodosius died at Mediolanum (Milan).