That’s what the new film is about
Around 200 years before the well-known events of Frodo, Aragorn and Gandalf, the animated film “The War of the Rohirrim” is about the story of the Rohan king Helm Hammerhand. He is a predecessor of Théoden (Bernard Hill, 78) known from “The Lord of the Rings: The Two Towers” (2002). After Helm kills the father of Dunlander Wulf with a single blow, he gathers an army and conquers large parts of Rohan. King Helm and his men then retreat to the castle of Helm’s Deep, later named after him, where Théoden faces the hordes of Saruman’s Uruk-hai in the second “Lord of the Rings” film with Aragorn, Gimli and Legolas.
Wulf’s army, made up partly of mercenaries, then besieges Helm’s Deep while combatants on both sides suffer the so-called Long Winter, a five-month period during which Rohan lies buried under a thick blanket of snow.