Which Tolkien work was published posthumously in 1977 as a collection of mythic histories?

The Silmarillion was published posthumously in 1977 as a collection of mythic histories.

J.R.R. Tolkien died in September 1973, leaving behind decades of manuscripts about the creation and early ages of his imagined world. His son Christopher Tolkien edited those materials into The Silmarillion, which George Allen & Unwin published in 1977. The book was not a conventional novel and included material in several narrative and historical styles.

Its sections cover the creation of the world, the Valar, the Elves, the wars against Morgoth, the tragedy of Túrin Turambar, the fall of Gondolin, the history of Númenor, and the Rings of Power. These stories provide much of the deep background later referenced in The Lord of the Rings.

The work is sometimes mistaken for Tolkien’s unfinished draft of The Lord of the Rings, but it is a separate mythology. It also differs from The History of Middle-earth, Christopher’s later twelve-volume documentary series about the development of his father’s writings.