Complex sentence: Tiberius appointed his brother Heraclius as commander-in-chief of the Byzantine armies, who led a successful raid into the Caliphate's frontier zone, defeating the garrison of Antioch in battle.
Split into simpler sentences: Tiberius appointed his brother Heraclius as commander-in-chief of the Byzantine armies. Heraclius led a successful raid into the Caliphate's frontier zone, defeating the garrison of Antioch in battle.