Assault on Helgrind is the 3rd chapter of Brisingr. It takes place from the view of Eragon. Part of it was released as an excerpt known as Light and Shadow in the back of the paperback issue of Eldest before Brisingr was released.
Saphira dove into Helgrind after discovering the illusion-concealed entrance with Eragon and Roran. Saphira and the Lethrblaka fought while Eragon and Roran battled the Ra'zac, Roran with his hammer and Eragon with his hawthorn staff (as he lost Zar'roc to Murtagh during the Battle of the Burning Plains) against the "pale blades of an ancient design". Eragon's face is burned by a Seithr oil-coated arrow but he heals it with magic before he and Roran look for Katrina.