Complex sentence: After the battle at Helm's Deep the Dunlendings were allowed to live by the Rohirrim, on the condition that all hostilities cease, and they retreated behind the Isen river again.
Split into simpler sentences: After the battle at Helm's Deep, the Rohirrim allowed the Dunlendings to live. The Rohirrim required that all hostilities cease, and that the Dunlendings retreat behind the Isen river again.