Short answers, pulled from the story.
The Battle of Evermore was written by Jimmy Page and Robert Plant at Headley Grange. Page wrote it in a single sitting after picking up John Paul Jones's mandolin, despite never having played mandolin before.
The Battle of Evermore directly references the Dark Lord and the Ringwraiths, and mentions war, bows, magic runes, and a dragon of darkness. Fans and scholars have often identified the battle in the song as the Battle of the Pelennor Fields, and the "Queen of Light" has been linked to the elf-queen Galadriel.
Sandy Denny was a former member of the British folk rock group Fairport Convention. Robert Plant invited her to duet on The Battle of Evermore because he felt the song needed a second voice; she sang the role of the town crier while Plant sang the narrator.
Sandy Denny was given the symbol of three pyramids on the Led Zeppelin IV sleeve to thank her for her contribution. The four members of Led Zeppelin each had their own symbols on the album, and Denny was the only guest to receive one.
The Battle of Evermore appears on Led Zeppelin's untitled 1971 album, commonly known as Led Zeppelin IV. It is the only Led Zeppelin studio recording to feature a guest vocalist.
Plant performed The Battle of Evermore regularly in spring and summer 2008 with Alison Krauss on their tour of the United States and Europe, supporting their 2007 album Raising Sand. He also sang the song with Fairport Convention and Kristina Donahue at Fairport's Cropredy Convention on the 9th of August 2008.