How did I not know this before?!

From Wiki

"The Battle of Evermore" is a folk duet sung by Robert Plant and Sandy Denny, included on Led Zeppelin's untitled 1971 album, commonly known as Led Zeppelin IV. The song's instrumentation features acoustic guitar and mandolin playing, while the lyrics allude to J. R. R. Tolkien's fantasy novel The Lord of the Rings.

Jimmy Page wrote it wuile messing about on John Paul Jones's mandolin