Passage: Jovinus' end came after the Visigoths under Ataulf left Italy (at Priscus Attalus' advice), ostensibly to join him, carrying with them as hostages the ex-emperor Attalus and Galla Placidia, Honorius' half-sister. Then Ataulf attacked and killed Sarus, who had also come to support Jovinus. Jovinus, offended at this act, then failed to consult Ataulf when he elevated his brother Sebastianus as co-emperor. Insulted, Ataulf allied his Visigoths with Honorius, and they defeated Jovinus' troops. Sebastianus was executed. Jovinus fled for his life, but was besieged and captured in Valentia (Valence, Drôme) and taken to Narbo (Narbonne), where Caius Posthumus Dardanus, the praetorian prefect (governor) in Gaul, who had remained loyal to Honorius, had him executed. Jovinus' and Sebastianus' heads were afterwards sent to Honorius and mounted on the walls of Ravenna (before being passed on to Carthage, where they were put on permanent display with the heads of four other usurpers).

Question: What is a praetorian prefect?
governor