Complex sentence: Skilurus died during a war against Mithridates, a decisive conflict for supremacy in the Pontic steppe in which the Scythians were defeated (ca.108 BC).
Split into simpler sentences: Skilurus died during a war against Mithridates, a decisive conflict for supremacy in the Pontic steppe. Soon after his death, the Scythians were defeated by Mithridates (ca.108 BC).