Complex sentence: A church synod, or council, was held in Rome in 313, followed by another in Arles in 314, the latter presided over by Constantine while still a junior emperor (see Tetrarchy).
Split into simpler sentences: A Church synod, or council, was called in Rome in 313 followed by another in Arles in 314. The latter was presided over by Constantine while he was still a junior emperor (see Tetrarchy).