What is the title of J.R.R. Tolkien’s posthumously published 1977 Middle-earth legendarium?

J.R.R. Tolkien’s posthumously published 1977 Middle-earth legendarium is titled The Silmarillion.

The book was edited and arranged by Tolkien’s son Christopher Tolkien and published four years after the author’s death. It presents creation myths, histories, tales, and genealogies reaching from the beginning of Tolkien’s world through the First Age and beyond.

Its central jewels are the Silmarils, three radiant stones made by Fëanor. Their creation and theft drive much of the tragic history of the Elves, including the wars against Morgoth. The book also contains the Ainulindalë, Valaquenta, Quenta Silmarillion, Akallabêth, and Of the Rings of Power and the Third Age.

The Silmarillion is not a conventional sequel to The Lord of the Rings. It is a collected mythology assembled from unfinished and variant writings, which explains its more historical and archaic style.