In Norse Mythology, Gunnar is the son of Gjuki and Grimhild and the brother of Hogni, Guttorm, and Gudrun, and the king of Nibelung after his father's death. He wanted to marry a valkyrie named Brynhild, but she swore that she would only marry the man who would ride though the ring of fire surrounding the residence Odin placed her in and his friend Sigurd then won Brynhild for Gunnar by performing the feat in his shape. When Brynhild found out, she tried to convince him to kill Sigurd and he couldn't so he had one of his younger brothers kill him, although it isn't clear which one.
Gunnar had sworn an oath of brotherhood to Sigurd which is why he was unable to kill him. He permitted Sigurd to marry his sister, Gudrun, and Sigurd aided him in a number of wars. After Sigurd's death, Gunnar and Hogni kept Sigurd's treasure and Gunnar married Glaumvor and married Gudrun off to Atli.
Atli found out that Gunnar had Sigurd's treasure, captured him and Hogni when they came to visit, and threw Gunnar in a pit of serpents when he refused to reveal the location of the treasure. Gudrun threw Gunnar a harp and he played it with his toes until all but the last snake fell asleep, but the last snake stung him in the liver and he died. The story of his death can be found in Dráp Niflunga from the Poetic Edda.