Fenrir is a very big and strong wolf from Norse mythology. He was born to the god Loki and a giantess, or a very tall woman. Fenrir was very special, because even as a little baby wolf, he was already huge and very strong.
As Fenrir grew older and bigger, the other gods became afraid of him. They knew that he could cause a lot of trouble if he wasn't kept in check. So they decided to put him in chains to make sure he couldn't run wild and hurt anyone.
But there was a problem. Fenrir was so powerful that no chain could hold him. So the gods had to make a special chain that was very strong and magical. They called it Gleipnir.
When they tried to put Gleipnir on Fenrir, the wolf got suspicious. He knew that the gods were up to something, and he didn't want to be trapped. So he asked the gods to prove that the chain was really strong.
The god Tyr volunteered to do this. He put his hand in Fenrir's mouth as a sign of good faith, so that Fenrir would trust them. Then the gods put the chain on Fenrir.
And it worked! Fenrir couldn't break the chain. He struggled and wriggled, but he was stuck.
The gods were relieved, and they knew that they had done the right thing. But Fenrir was furious. He knew that the chain meant that he was trapped forever, and he hated the gods for doing this to him.
This is the story of Fenrir, the big, strong wolf from Norse mythology who was so powerful that the gods had to use magic to keep him under control.