Complex sentence: Ammianus Marcellinus (325 330--after 391) was a fourth-century Roman soldier and historian who wrote the penultimate major historical account surviving from Antiquity (preceding Procopius).
Split into simpler sentences: Ammianus Marcellinus (325 330--after 391) was a fourth-century Roman soldier and historian. He wrote the penultimate major historical account surviving from Antiquity (the last was written by Procopius).