Glaurung is the dragon defeated by Túrin’s sister Nienor’s husband in Tolkien’s The Children of Húrin.
Glaurung is the first fully named dragon in Tolkien’s Middle-earth mythology. He serves Morgoth, the great enemy of the First Age, and is associated with the destruction of Nargothrond. His power includes not only physical strength and fiery breath but also a hypnotic gaze and the ability to deceive.
Túrin Turambar ultimately kills Glaurung with the sword Gurthang. The dragon’s death reveals the truth about Nienor’s identity and helps bring the tragedy of Túrin’s family to its devastating conclusion. The wording of the question refers to Túrin’s relationship with Nienor rather than naming Túrin directly, because the episode is part of the story’s tangled family tragedy.
Glaurung is often confused with Smaug, the dragon of The Hobbit. Smaug belongs to a later age and is killed by Bard the Bowman. Ancalagon the Black is another First Age dragon, but he is associated with the War of Wrath.