Which J.R.R. Tolkien story was published posthumously in 1977 by Christopher Tolkien?

The Silmarillion was published posthumously in 1977 by J.R.R. Tolkien’s son Christopher Tolkien.

J.R.R. Tolkien died in 1973, leaving a large body of unfinished and variant writings about the history of Middle-earth. Christopher Tolkien edited these materials into a publishable volume with assistance from the fantasy writer Guy Gavriel Kay. The book appeared four years after Tolkien’s death.

The Silmarillion presents the mythology and history underlying The Hobbit and The Lord of the Rings. It begins with creation stories in the Ainulindalë and continues through the history of the Valar, the Elves, the Silmarils, and the First Age wars against Morgoth. It also includes material about Númenor and the Rings of Power.

Because the source manuscripts contained contradictions and revisions, the book is an editorial reconstruction rather than a novel completed in final form by Tolkien. Unfinished Tales, published later in 1980, is a separate collection of narrative fragments and commentary.