Who is Fenrir?
Fenrir is the wolfson of Loki and the giantess Angrboda. He was born along with Hel, Goddess of Helheim and Death, and the Midgard Serpent. When he was born, the Gods of Asgard heard a prophecy that Fenrir would grow to an unimaginable size and strength, and bring about destruction. To prevent this, they took Fenrir away from Loki and kept him in Asgard. They attempted to tame him, and placed the God Tyr in charge of feeding him.
His story, as summarized by Hexen_Feuer: "When Fenrir grew in size, the gods all [became worried that he would fulfill the prophecy and they devised a plan to bind him. They produced a chain and presented it to Fenrir, challenging that he wasn't strong enough to break the chain.] Fenrir accepted this challenge, and broke through them with ease. The gods then turned to a magical chain fashioned from six strange materials by the dwarves. Those materials consisted of: fishes breath, a cats footstep, roots of a mountain, a woman's beard, the sinew of a bear, and a bird's spittle. The magical chain, Gleipnir, was so thin and soft that many would doubt it's strength.
Fenrir was less eager to break the chain, he said there was no pride in breaking such a chain. Fenrir agreed, though suspicious of treachery, he asked for the gods, as a token of good will, to place a hand between his jaws, and only Tyr agreed. Fenrir struggled with the thin chain, and no matter how hard he tried, he could not break it, so he bit off Tyr's hand in an act of revenge. The gods, being very pleased with the results, carried fenrir off and tied him to a rock, Gioll, which was a mile within the Earth. They then put a sword between his jaws to keep him from biting. [Two rivers of blood ran him Fenrir's mouth and formed Van(Hope) and Vil(Despair)] .
On the day of Ragnarok, Fenrir 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 him by killing him. "
Honoring/Working With Fenrir:
Fenrir is not typically worshipped, honored or worked with by most Heathens. However, Rokkatruars (who honor the Rokkr pantheon, of which Fenrir is a part of) honor Fenrir and include him in their workings. Part of the Rokkatru ethics/lessons is dedicated to Fenrir:
Fenris's Rule: Shadow. Learn to love and find sacred all the parts of yourself, even the darknesses. Honor them by making a safe place for them, where they can neither be harmed nor harm others.
Fenrir often teaches the lesson of morality, and accepting the "wolf within" so to speak. In many ways his nature reflects our own. He teaches the lesson of being yourself, as he would not be bound if he could chage his ways- but he would never change his ways because he is proud of who he is.
Some Heathens like to include Fenrir in practices such as Berserking. Berserking can be an excellent way of dealing with pent up emotions and anger, of which Fenrir is very accustomed to feeling. Channeling Fenrir is not unheard of, though it is very uncommon in our practice. As Raven K says, " Horsing him is usually done as an offering, as tribute, and to "feed" him. To do it for any reason outside of that is not wise. He enjoys voluntary pain. He enjoys fresh meats and blood. He also enjoys the emotion of fear, like Hel.He is often called the "God of Last Resorts" and he is called that for a reason. To work with Fenrir is to unleash a part of yourself that most people spend their lives trying to bury or get rid of. He is extremity, the very definition of it. And he often wears his horses out. "
Offerings to Fenrir vary, since there isn't anything traditional. Normally he is not made an altar, rather you use something in nature, something primal, to give offerings on. I personally have a tree stump in my yard that I've dedicated to him. Offerings to him include freshly killed animals, raw meats, blood, red candles, red cloth, emotions such as pain or fear, and things related to wolves.