The prequel film is set 183 years before the original trilogy and revolves around the House of Helm Hammerhand, the legendary King of Rohan.

The Lord of the Rings: The War of the Rohirrim
A Dunlending lord, consumed by vengeance, launches a surprise attack on Helm Hammerhand, the King of Rohan, and his people, who must rally to defend their stronghold in the ancient fortress of the Hornburg.