Complex sentence: Narbona held back his forces in hiding on both sides of the pass, saying that when the time was right and not before they would cut the long file of men into small pieces, like cutting a long tree trunk into firewood.
Split into simpler sentences: Narbona held back his forces, who hiding on both sides of the pass. He told them that when the time was right and not before they would cut the long file of men into small pieces, like cutting a long tree trunk into firewood.