J.R.R. Tolkien’s unfinished mythology published posthumously by his son Christopher in 1977 is The Silmarillion.
The Silmarillion presents the creation of Tolkien’s world, the struggles of the Valar and Elves, and the major histories of the First Age. It includes the tale of the Silmarils, precious jewels made by the Elf Fëanor, whose theft leads to wars, oaths, exile, and tragedy.
Tolkien worked on related versions of these stories for much of his life, but he did not complete a final form for publication. After his death in 1973, Christopher Tolkien edited material from manuscripts and arranged it into the book published in 1977.
The Silmarillion is often mistaken for a conventional sequel to The Lord of the Rings. It is instead a collection of mythic histories, genealogies, and narratives that provide the deep background of Middle-earth. Unfinished Tales, published later in 1980, contains additional but separately edited material.