Fenrir is one of the three children of Loki and Angrboda, along with her ‘big brother’ Jörmungandr, and her ‘little sister,’ Hel. Not long after their birth, a prophecy was made foretelling that the three would cause great trouble for the gods, and thus the three were separated at an early age. Jörmungandr would be thrown into the sea, Hel would be sealed in her own realm and tasked with watching over the dishonored dead, and Fenrir was taken into custody by the Aesir.
Understandably angry at being taken away from her family, Fenrir refused all comforts the gods provided for her and spent most of her days in solitude, until Tyr, the god of war and the only god who pitied her sorrow extended his hand in friendship to her. It took weeks of effort, but eventually she opened her heart to him as he had to her, and the two spent many happy days together.
He allowed her a small measure of freedom in the local forest, where she could be not a prisoner of the gods, but just another animal, and it was there that she learned how to hunt, how to fight, and came to love the natural world in all its beauty.
In time she would find a mate and became mother to the twin wolves Skoll and Hoti. Everything seemed like it was finally taking a turn for the best, and indeed, if things had remained as they were, she may never have become the Wolf of Ragnarok at all. Alas, fate had other plans for her.
The gods had noticed she was growing very fast, indeed by the time she had torn Leyding asunder she was already the size of a large house. Remembering the prophecy, they sought to contain the threat before it could grow any further. Thus, they commissioned the dwarves to create a mighty chain, Leyding, and had Tyr bring it before her. He was reluctant, but fate itself had decreed Fenrir be chained, and as he was bound to it as much as any other Norse deity was, he had no choice but to obey.
Tyr found Fenrir in the midst of play and showed her the chain, posing it before her as a fun test of strength: if she could break the chain, she wins! Fenrir tested the chain, decided it was within her ability to break, and accepted the challenge. Sure enough, within seconds the chain cracked, bent, and finally broke under Fenrir’s incredible strength.
Not about to give up, the gods ordered the creation of a second set of chains that was twice as strong, Dromi, and repeated the same process. But the second set broke as quickly as the first! The gods become terrified that if Fenrir were not restrained soon, the task would become utterly impossible, and so Odin himself sent his messenger Skirnir to Svartalfheim to convey the urgency and weight of this task directly to the dwarves.
And so, the dwarves rationalized: “If restraining Fenrir truly is impossible, than impossibility is what shall restrain her!” And so, against all odds and all logic, they collected six ‘impossible ingredients’ to forge their great masterwork: the sound of a cat's footfall, a woman’s beard, a mountain’s roots, a bear’s sinews, a fish’s breath, and a bird’s spittle.
The gods took these ultimate chains to Fenrir at the island of Lyngvi and once again challenged her to break free from them. But Fenrir sensed these chains were of a very different sort from the others. They seemed so fragile, and yet every instinct in her body was screaming at her to refuse, to run, to fight her way out if need be! Tyr sensed her distress, and though it pained him to do so, he, the one true friend among the Aesir she had, calmed her with sweet nothings and laid his hand within her mouth, promising that she would surpass this challenge, and that if she did not, his hand would be forfeit to her.
Shocked at this declaration, she agreed to test the strength of the chains. At the moment the chains fastened tight, she strained with all her might. But the chains only seemed to grow stronger as she tore at them. No, they seemed to be feeding off her own might and using it against her! She panicked, and in her panic she bit down on Tyr’s hand and begged to be set free from the chain. But the gods only jeered at her, spat on her, called her a monster, dragged her to a great rock and tied her there.
She called out to her best friend, begging him to make them stop, to tell them she was no monster, to explain what all this was about, but he would not answer, or even look her in the face. At that moment, Fenrir broke. She screamed curses to the Aesir, declaring she would kill them all for this betrayal, along with all they cared about, and see Asgard fall to ruin. And to Tyr especially, she vowed he would never be forgiven, and that his death would be the slowest and most painful of all.
Even with a great sword jamming her mouth open, she continued to curse them, and when they had all left her alone on that island and she had finally ran out of curses to shout, only then did she finally break down and cry in despair at all she had lost. This despair and the isolation that accompanied it eventually drove her mad with grief and hate. It is told that the river Vįn was formed from her saliva, but what is not written in any legend is that just as much tears went into that river.
In the end, even Gleipnir would prove unable to hold her ever growing strength forever, and she would break free from her chains to fight alongside her family at Ragnarok, and their army would kill many gods that day. Fenrir herself managed to devour Odin before dying at Vķšarr’s spear, released from her torment at long last.