Complex sentence: The Surdan and Varden armies together fought the Empire's forces in the Battle of the Burning Plains an area that lies immediately northwest of Surda, during the battle Eragon and Saphira faced the Empire's Rider Murtagh and his dragon Thorn.
Split into simpler sentences: The Surdan and Varden armies together fought the Empire's forces in the Battle of the Burning Plains, an area that lies immediately northwest of Surda. During the battle, Eragon and Saphira face the Empire's Rider Murtagh and his dragon Thorn.