The first Middle-earth story published posthumously by J.R.R. Tolkien’s son Christopher was The Silmarillion.
The Silmarillion was published in 1977, four years after Tolkien’s death. Christopher Tolkien edited material from his father’s extensive papers and worked with the writer Guy Gavriel Kay to prepare the text for publication. It presents the mythology and history of Arda from its creation through major events of the First Age.
The book includes the creation story, the Ainulindalë, the account of the Valar, and narratives involving the Silmarils, Morgoth, Elves, Men, and the fall of Númenor. Much of this material existed in multiple versions, so Christopher had to make editorial decisions.
Later posthumous publications included Unfinished Tales and separate editions of major narratives such as The Children of Húrin and The Fall of Gondolin. Those books are not the first posthumous Middle-earth publication.