The Lord of the Rings influenced The War of the Rohirrim

The film is built out of Appendix A, where Tolkien gives the House of Eorl a few paragraphs: Helm Hammerhand, the winter siege of the Hornburg, Wulf's attack, and the deaths that end Helm's line. That is the entire source. The appendix names the events and almost nobody in them speaks; Helm's daughter, who the film is about, is not given a name by Tolkien at all. Filed LOW for exactly that reason. Everything that makes it a story, and the protagonist herself, was written for the screen around a skeleton of published fact. That is a real adaptation and a thin one.