Complex sentence: Leading the armies of the Dark with the aid of six lieutenants known as the Fallen Lords, they turned the once-fertile farmlands surrounding Muirthemne into a barren desert known as "The Barrier".
Split into simpler sentences: Leading the armies of the Dark with the aid of a group of lieutenants, they became known as the Fallen Lords. The once-fertile farmlands surrounding Muirthemne became a desert known as "The Barrier".