Which Tolkien work, published posthumously in 1977, presents the mythology of Arda?

Tolkien’s work published posthumously in 1977 that presents the mythology of Arda is The Silmarillion.

The Silmarillion was edited and published by Christopher Tolkien four years after J.R.R. Tolkien’s death. It is not a single conventional novel; instead, it is a collection of related narratives, histories, genealogies, and legends describing the creation and history of Tolkien’s imagined world.

The book begins with the Ainulindalë, a creation account, and continues through the Valaquenta and the Quenta Silmarillion. The latter focuses on the Silmarils, the great jewels made by Fëanor, and the wars caused by their theft. Later sections cover the Second and Third Ages in more compressed form.

Because Tolkien revised many of these stories across his lifetime, Christopher Tolkien had to make editorial choices when preparing the posthumous volume. Readers sometimes mistake The Silmarillion for a finished manuscript in exactly the same form as The Hobbit, but its publication history is substantially more complex.