Complex sentence: Eventually, Gronw agrees to receive the blow on the condition that he may place a large stone between himself and Lleu, who allows him to do so before throwing the spear with such strength that it pierces the stone, killing his rival.
Split into simpler sentences: Eventually, Gronw agrees to receive the blow on the condition that he may place a large stone between himself and Lleu. Lleu allows Gronw to do so, then throws the spear with such strength that it pierces the stone, killing his rival.