Complex sentence: Kellen realizes that they mean to go around the army and burn Ysterialpoerin, so he takes his men to go and stop them before they destroy the city and finds that the shadow elves have a new fire that cannot stop burning by any means.
Split into simpler sentences: Kellen realizes that they mean to go around the army and burn Ysterialpoerin. So he and his men go and stop them before they destroy the city and learn that the shadow elves have a new fire that cannot stop burning by any means.