The Silmarillion was published posthumously in 1977 after being edited by Christopher Tolkien.
J.R.R. Tolkien died in 1973, leaving a large and complex body of writings about the history, languages, peoples, and legends of Middle-earth. Christopher Tolkien arranged and edited material from these papers to create a coherent published work. The result presents mythic history beginning with the creation of the world and continuing through the First Age and later periods.
The book is not a conventional sequel to The Lord of the Rings. Its central sections include the Valaquenta, the Quenta Silmarillion, the Akallabêth, and material concerning the Rings of Power. Because Tolkien left multiple versions and unfinished passages, Christopher’s editorial role was substantial, which distinguishes this publication from a completed novel prepared by J.R.R. Tolkien himself.