Complex sentence: When Galerius himself invaded Italy to suppress Maxentius and Maximian, the former ordered Severus's death: he was executed (or forced to commit suicide) on 16 September 307.
Split into simpler sentences: When Galerius himself invaded Italy to suppress Maxentius and Maximian, the former ordered Severus' death. He was executed (or forced to commit suicide) on 16 September 307.