Complex sentence: During the later Third Age the Pass became dangerous again because of the Orcs, only with the War of the Dwarves and Orcs, which nearly wiped out all Orcs of the mountains, did it become safe again.
Split into simpler sentences: During the later Third Age the Pass became dangerous again because of the Orcs. Only with the War of the Dwarves and Orcs, which nearly wiped out all Orcs of the mountains, did it become safe again.