Which Tolkien work was published posthumously in 1980 as Unfinished Tales?

The Tolkien work published posthumously in 1980 was Unfinished Tales of Númenor and Middle-earth.

Christopher Tolkien edited and published Unfinished Tales in 1980, seven years after J.R.R. Tolkien’s death. The collection contains narratives and notes from different periods of Tolkien’s writing, covering subjects from the First Age through the events surrounding The Lord of the Rings. It includes material about Númenor, the Istari, Galadriel, and the history of the Rohirrim.

The title is important because many pieces are incomplete, exist in multiple versions, or require editorial explanation. Christopher Tolkien supplied commentary to identify relationships among drafts and to explain where the texts do not form a single finished narrative.

The book is not the same as The Silmarillion, which was also edited by Christopher Tolkien but published in 1977. Nor is it the later standalone novel The Children of Húrin, published in 2007. Unfinished Tales is best understood as a curated collection of unfinished legendarium material rather than one continuous story.