Complex sentence: His is the last major historical account of the late Roman empire which survives today: his work chronicled the history of Rome from 96 to 378, although only the sections covering the period 353-378 are extant.
Split into simpler sentences: His is the last major historical account of the late Roman empire which survives today. His work chronicled the history of Rome from 96 to 378, although only the sections covering the period 353-378 are extant.