Who is Glaurung from Unfinished Tales of Numenor and Middle-earth and what is their importance?

Glaurung is the great dragon from "The Tale of the Children of Hurin." Glaurung is a servant of Morgoth sent to destroy Nargothrond. Glaurung casts a spell over Nienor that causes her to forget her entire life up to the present. He lifts the spell just as he dies, and lets Nienor see that she has married her brother and now carries his child.