The first completed Middle-earth story published by J.R.R. Tolkien’s son Christopher was The Silmarillion.
Christopher Tolkien edited The Silmarillion from his father’s posthumous papers, and the book appeared in 1977. It was the first major published collection of Tolkien’s mythology to follow his death, presenting material that reaches from the creation of Eä through the First Age and beyond.
The wording matters because Christopher later published many other works from the archive. Unfinished Tales appeared in 1980, while The Book of Lost Tales was published in two volumes in the 1980s as part of The History of Middle-earth. The Children of Húrin was published in 2007. Christopher’s editorial work made extensive versions of his father’s legendarium available, but The Silmarillion remains the landmark first publication of that posthumous project.