Under the guidance of Regin, Sigurd killed Fafnir, Regin's older brother that had killed their father Hreidmarr and monopolized his treasure. Afterward, Fafnir's heart was cut out and roasted for Sigurd and Regin to eat. According to Poetic Edda, Regin used Ridill to rip out Fafnir's heart. But in the Volsunga saga, it is Sigurd who used Ridill and cut out the heart at that occasion.
Incidentally, in the Skáldskaparmál the name of Regin's sword is Refil instead.