The Volsunga Saga

Anderson wrote a saga rather than a novel about one, and the borrowings are structural:

• A sword broken and reforged as the hinge of the plot, which is Sigurd's Gram
• A blade that must kill whenever it is drawn and turns on its owner, the Tyrfing curse
• A changeling, a doomed hero raised away from his kin, and incest discovered too late, all of which the Volsungs supply
• The saga voice itself: flat, fast, fatalistic, and offering no interiority

Published the same year as The Fellowship of the Ring and drawing on the same Norse source Tolkien taught, which is why the two books read as opposite answers to one question.

Tolkien was a professional philologist working on exactly this material, and the Silmarillion takes its architecture from it: a cursed treasure that destroys everyone who holds it, a dragon guarding hoard and speaking with its victim, a hero who kills that dragon from below, and a family destroyed across generations by an oath. Túrin Turambar is Sigurd crossed with Kullervo, and Tolkien said so;

Smaug was directly inspired by Fáfnir, the intelligent, treasure-hoarding dragon whose greed defines the Völsunga Saga.

The broken sword Gram, reforged for Sigurd, is a clear precursor to Narsil/Andúril, the broken sword reforged for Aragorn. Strong heros identity inspired by the saga too!