Fenrir (the fen dweller), also known as Fenrisulfr (the fen wolf) or Hróðvitnir (famous wolf), is the son of Loki.
When the gods found out about Loki's children had a look upon them, they were horrified. The first, Hel, was half-living and half-dead and was taken to Hel by Odin where he gave her power over nine worlds and put her to take care of the dead who died of sickness, old age or simply where not elected for Valhalla or Folkvangr. The second child was Jörmungandr, who was a snake and thrown it into the ocean, hoping it was drowned. Instead, the snake grew so large that it encircled all of Midgard and finally started to eat it's own tail.
The third child was Fenrir. He looked like a little puppy, and since Odin already had two wolves as a companion, the gods brought the wolf cub to Asgard. But the cub grew and became larger and more fearsome than any wolf or dog. Soon, the gods became afraid of it. Only Týr had the courage to feed it. The gods saw that Fenrir was growing rapidly every day and the prophecies foretold that Fenrir was destined to cause great sorrow among them, so they decided to bind him. First came Odin with the thick rope Leyding and suggested that the wolf should test his strength with it. Fenrir broke the rope with ease. The gods then came with a new fetter called Dromi. They told Fenrir that he would achieve great fame if he was able to break the chain they had brought. Even this fetter was broken.
The god Freyr sent a servant to the dwarves in Niðavellir to see if they could make a fetter that could trap the beast. From the sound of a cat's footfall, the beard of a woman, the roots of a mountain, the sinews of a bear, the breath of a fish and the spittle of a bird the dwarfes formed the fetter Gleipnir. The gods brought the fetter to Fenrir and asked him to test his strength with it. The gods said that not even they could break the fetter. When Fenrir saw the silken thread, he who had his father's cunning said that such a small ribbon would not bring any man any glory for breaking and said that he feared a treasonous intent with such a test. Fenrir than asked that to prove that the gods where not trying to trick him one of them should put their hand in his mouth. Týr volunteered to put his left hand in Fenrir's mouth. Then the gods placed Gleipnir around Fenrir's neck and the beast found himself stuck. The gods laughed at him, all but Týr, for he was now one-handed.
Through out Vikings, Fenrir is often mentioned in their tales.