According to legend, Fenrir was the son of Loki, an evil god, and a giantess named Angrboða. The gods knew that Fenrir was a threat, as it was said that he was so big that he could swallow the sun and the moon.
To prevent Fenrir from wreaking havoc, the gods decided to bind him with a magical chain called Gleipnir. However, Fenrir was so strong that he managed to break the chain. The gods tried to bind him again with an even stronger chain, but Fenrir managed to break it again.
Finally, a god named Týr offered his hand as a guarantee to convince Fenrir to allow them to bind him with an iron chain. Although Fenrir was reluctant, he finally agreed, but once he was bound, he fought against the chain and tightened it until it broke, which allowed him to free himself and become a great threat to the gods.