Complex sentence: At the same time a new general was found to support Honorius, the future Constantius III, who arrived at Arles, put Gerontius to flight, then took over the siege of Constantine in Arles.
Split into simpler sentences: At the same time a new general was found to support Honorius. The future Constantius III, who arrived at Arles, put Gerontius to flight then took over the siege of Constantine in Arles.