Complex sentence: That eastern people, for so long a dreaded enemy of Gondor, never challenged her again, and the battle where Eärnil won his success was named after the Wainrider camp where it had taken place: the Battle of the Camp.
Split into simpler sentences: That eastern people, for so long a dreaded enemy of Gondor, never marched against Gondor again. The battle was named after where it had taken place: the Battle of the Camp.