Which Tolkien work was published posthumously in 1977 under Christopher Tolkien’s editorship?

The Silmarillion was published posthumously in 1977 under Christopher Tolkien’s editorship. J.R.R. Tolkien died in 1973, leaving behind a large body of mythology, drafts, revisions, and unfinished narratives concerning the history of Arda.

Christopher Tolkien organized this material into a publishable volume, with assistance from the writer Guy Gavriel Kay. The resulting book was published on 15 September 1977. It begins with the creation of the world and proceeds through the history of the Valar, the Elves, the wars against Morgoth, and the events that lead toward the later histories of Middle-earth.

The title refers especially to the three great jewels called the Silmarils, created by the Elf Fëanor. Their theft by Morgoth drives much of the First Age narrative. The book is not a conventional sequel to The Lord of the Rings; it is closer to a collection of myths, chronicles, genealogies, and legends presented in a formal, elevated style.

Because Tolkien continually revised his mythology, The Silmarillion reflects editorial decisions rather than one final manuscript completed by its author. Christopher later documented those decisions in The History of Middle-earth.