Regin forged a marvelous sword for Sigurd, but it quickly broke. Sigurd found his father's (Sigmund) sword, Gram, and had it fixed and reforged by Mimir and used it to kill Fafnir. He gained wisdom from licking the dragon's blood because Fafnir could talk to birds. Sigurd, who had discovered that Regin was planning on killing him to get the gold from the birds, killed his stepfather and took the gold.
The Thiorekssaga relates a slightly different tale, with Regin as the dragon and Mimir as his brother and foster father to Sigurd.