Which Tolkien story features the dragon Glaurung as its principal dragon?

The Children of Húrin features Glaurung as its principal dragon.

Glaurung is a powerful dragon in Tolkien’s First Age mythology and is called the Father of Dragons. He serves Morgoth, the first Dark Lord, and plays a central role in the tragic story of Túrin Turambar and his sister Niënor. Glaurung is notable not only for physical power but also for deceptive speech and the ability to dominate the minds of those who face him.

The Children of Húrin was published posthumously in 2007, edited by Christopher Tolkien from writings his father had produced across many decades. The story had appeared in shorter or incomplete forms in earlier publications, including The Silmarillion and Unfinished Tales.

Glaurung should not be confused with Smaug, the dragon in The Hobbit. Smaug belongs to the later Third Age, whereas Glaurung belongs to the First Age and the wars against Morgoth.