Complex sentence: The Parthians rushed to attack, whether this order came from Pacorus or was a spontaneous charge is unknown, in either case Ventidius ordered his troops who had the advantage of high ground to attack the horse-archers advancing up the slope.
Split into simpler sentences: The Parthians rushed to attack-whether this order came from Pacorus or was a spontaneous charge is unknown. In any case, Ventidius ordered his troops, who had the advantage of high ground, to attack the horse-archers advancing up the slope.