Helm’s story can be found in the appendix to “The Return of the King”. In the addendum to the third volume of the “LOTR” trilogy, Tolkien takes a handful of pages to give us a brief history of Rohan. It starts with Eorl, the founder of the Equine Kingdom, leading his people out of northeastern Middle-earth to save Gondor from an invasion. After that, the Steward of Gondor offers the saved region to the Horse Lords as a gigantic thank you for saving their skins.
Once Rohan was founded, the descendants of Eorl continued to rule the young kingdom, with a king following him down from father to son until the ninth king, Helm, took over. We don’t have much information about Helm’s early years. Instead, Tolkien presents him with the dramatic line “Of the Kings of the Mark between Eorl and Theoden, most said of Helm Hammerhand. He was a dark man of great strength.”
From there he dashes into Helm’s story, starting with the king already being middle-aged and busy ruling his sprawling kingdom. The fact that Helm doesn’t get much of an introduction can be disappointing, but it’s more than made up for by the rest of his over-the-top epic story – one that begins with hawkish discrimination.
You see, Helm has a bit of prejudice against Rohan’s neighbors, a group called the Dunlendings. They were moved when Rohan arrived in the area, and at this point they mostly live in the north of the country, near an ancient Gondorian fortress called Isengard (and no, Saruman does not live there yet, although he shows up shortly after the end of Helm’s story).
Either way, Helm isn’t a fan of the Dunlendings, who have a long-standing conflict with the Rohirrim who drove them from their homes a few hundred years earlier. This sets the stage for the conflict that defines Helm’s entire career.