Erkenbrand was the lord of the Westfold of Rohan, the country at the western end of the White Mountains that held Helm's Deep and the Hornburg.1
When Saruman's armies came against Rohan the Riders of the West-mark held the Fords of Isen under King Théoden's son, Théodred. Théodred was killed in the first battle there, and in the second, with Erkenbrand now commanding the West-mark, the Riders were broken and scattered, so that when Théoden rode west Erkenbrand was missing and Helm's Deep was manned without him. Unfinished Tales gives the fullest account of the fighting at the Fords.2,1
Gandalf rode off on the eve of the battle on a swift errand, gathering what men he could find and sending some with Grimbold of Westfold to join Erkenbrand. At dawn on 4 March 3019, as Théoden rode out of the Hornburg, Erkenbrand, with about a thousand men on foot, and Gandalf came down on the besiegers from the hills, and Saruman's army broke and fled into the forest of Huorns that had grown up in the night.1,3
Sources
Citations name the chapter rather than the page, so they work in any edition. The colour of each note shows what kind of source it is.
- Published story The Hobbit and The Lord of the Rings as Tolkien published them.
- Edited posthumously The Silmarillion, Unfinished Tales and Tolkien's late writings on the Elder Days, assembled by Christopher Tolkien from his father's papers.
- Published story The Two Towers, Book III, ch. 7, “Helm's Deep”
- Edited posthumously Unfinished Tales, Part Three, V, “The Battles of the Fords of Isen”
- Published story The Two Towers, Book III, ch. 8, “The Road to Isengard”