The Silmarillion, published posthumously in 1977, describes the earlier history of Middle-earth.
The book was edited and arranged by Christopher Tolkien from his father’s extensive writings. It presents mythic and historical material reaching back to the creation of the world, the shaping of Arda, the awakening of the Elves, and the wars against Morgoth, the first Dark Lord.
Its central parts include the Valaquenta, the Quenta Silmarillion, the Akallabêth, and “Of the Rings of Power and the Third Age.” The title refers especially to the Silmarils, three precious jewels made by Fëanor whose theft drives much of the First Age story.
The Silmarillion is not a conventional sequel to The Lord of the Rings. It is a collection of legend-like narratives, genealogies, and historical accounts assembled from versions Tolkien revised throughout his life. Unfinished Tales and The History of Middle-earth are related posthumous publications but are different works.