Complex sentence: The city was attacked in "Eragon" by an army of Urgals, controlled by the shade, Durza, they delivered a fierce onslaught on the Varden and the dwarves, but were defeated when Eragon killed Durza.
Split into simpler sentences: The city was attacked in "Eragon" by an army of Urgals, controlled by the shade, Durza. They delivered a fierce onslaught on the Varden and the dwarves, but were defeated when Eragon killed Durza.