Which Tolkien work was published posthumously in 1977 from his mythology of the First Age?

The Silmarillion was published posthumously in 1977 as a collection of J.R.R. Tolkien’s mythology, especially stories of the First Age.

Tolkien worked on the material for decades, revising its stories, languages, and genealogies throughout his life. It includes the creation of the world, the Valar, the Silmarils, and the great conflicts of the First Age. Unlike a straightforward novel, it combines mythic narrative, annalistic sections, and related texts.

Christopher Tolkien edited the published volume after his father’s death, using manuscripts from different periods. The book is therefore not a single final manuscript in the same way as The Hobbit. Unfinished Tales, published later in 1980, presents additional incomplete narratives from Tolkien’s legendarium.