Fenris is a gigantic wolf. He is the oldest child of the trickster god Loki and the giantess Angrboda.

The god of war, Tyr, was the only one who dared to feed and take care of the wolf. While Fenris was still a pup the gods had nothing to fear, but when the gods one day saw how he had grown, they decided to render him harmless. However, none of the gods had enough courage to face the gigantic wolf, so they tried to trick him instead. They said the wolf was weak and that he could never break free when he was chained. Fenris accepted their challenge, and let himself be chained. The first time Fenris was chained, he broke through the fetters as if they were cobwebs. The gods challenged Fenris twice more, each time using stronger and larger chains. But Fenris was so strong he was still able to break them.

After that, the gods saw only one alternative left: a magic chain. They ordered the dwarves to make something so strong that it could hold the wolf, and the result was a soft, thin ribbon: Gleipnir. It was incredibly strong, despite what its size and appearance might suggest. The ribbon was fashioned of six strange elements: the noise made by the footfall of a cat; the roots of a mountain; a woman's beard; the breath of fishes; the sinews of a bear; and a bird's spittle.

The gods tried to trick the wolf again. Only this time Fenris saw how thin the chain was, and said that there was no pride in breaking such a weak chain. Eventually Fenris agreed, thinking that otherwise his strength and courage would be doubted. Suspecting treachery however, he in turn asked the gods for a token of good will: one of them had to put a hand between his jaws. The gods were not overly eager to do this, knowing what they could expect. Finally, Tyr who agreed to the task. The gods then chained the wolf with Gleipnir. No matter how hard Fenris struggled, he could not break free from this thin ribbon. When the gods refused to set him free he bit off Tyr's hand in revenge.

Being very pleased with themselves, the gods carried Fenris off and chained him to a rock, a mile down into the earth. They then put a sword between his jaws to prevent him from biting. On the day of Ragnarok, Fenris will break his chains and join the giants in their battle against the gods. He will seek out Odin and devour him. Vidar, Odin's son, will avenge his father by killing the wolf.

Also known as: Hrodvitnir ("mighty wolf"), Fenris, Fenrisúlfr