Which Tolkien family member edited the 12-volume History of Middle-earth?

Christopher Tolkien edited the 12-volume History of Middle-earth.

Published between 1983 and 1996, the series presents drafts, revisions, notes, and related writings from J.R.R. Tolkien’s papers. Christopher Tolkien arranged and annotated this material, showing how the mythology developed over many decades and how names, stories, and cosmology changed during composition.

Christopher was J.R.R. Tolkien’s third son and became the principal literary executor of his father’s unpublished work. His editorial project is not a single novel and should not be confused with The Silmarillion, which he edited and published in 1977 with material selected from Tolkien’s papers.

The History of Middle-earth is especially valuable because it reveals abandoned versions and competing ideas rather than presenting only the polished forms familiar from the published books. Its volumes therefore document Tolkien’s creative process as well as the fictional history of Middle-earth.