Complex sentence: He thinks nothing of razing entire towns and commands a great army; however, his lack of subtlety is his weakness, because it allows the Assassins who operate in the shadows to keep their eyes on him at all times.
Split into simpler sentences: He thinks nothing of razing entire towns, and commands a great army. But his lack of subtlety is his weakness, because it allows the Assassins, who operate in the shadows, to keep their eyes on him at all times.