Gramr is a weapon in Fire Emblem Heroes. It's a powerful blade that can best Fáfnir if only a member of the royal family of Niðavellir can wield it. In the Book V cutscenes, Reginn is shown to wield it, with the sword glowing. However, it has not been confirmed whether Reginn is royalty or not.
In Norse Mythology Gram is the personal sword of Sigurd that once belonged to his father Sigmund. The sword was used to kill the dragon Fáfnir and then was used to kill Sigurd's foster father Reginn when he attempted to kill him.