Alfadr (Odin) had thrown him to the bottom of the sea because a prophecy had announced that the children of Loki would decimate the Aes.
Since then he had grown up and bit his tail; he held the earth between his powerful rings and when he moved, he caused tidal waves. In fact, he was part of the cohesion of the earth.
During the Ragnarök, Jormungand will provoke a gigantic tidal wave by coming out of the sea to go with the Giants to fight the Ases and especially Thor. Thor will succeed in defeating him but he will not have taken nine steps to collapse mortally wounded by the poison instilled in his flesh by the snake.
The first meeting between the Ases and Jormungand took place in the great hall of the Utgarda-Loki Giant. In order to humiliate them, the king offered them some easy challenges. Utgarda-Loki suggested to Thor to lift a cat. As Thor lifted the animal, it grew in such a way that its legs always remained in contact with the ground, much to the amusement of all the Giants gathered. Later Thor learned that he had actually tried to lift Jormungand, who had taken on the appearance of a cat.
Another day Thor decided that it was high time to take on Jormungand.
He went to the port and tried to rent a boat. The only one he could find in almost good condition belonged to the Giant Ymir who was very afraid to go to sea when Jormungand was awake. Thor did not inform him of his plans and paid a more than reasonable amount. Then he took the thickest fishing line he could find.
At first Ymir fished for two whales, then Thor surreptitiously approached the place where Jormungand was and hung an ox head on his line as bait. Then he plunged her into the sea. It was not long before Jormungand took the bait.
He then began to pull with all his might. The snake was also pulling on its side and the small boat was pitching dangerously. He finally managed to bring out the monster's head and was about to give him a masterful blow with his Mjöllnir hammer when the terrified Giant cut the line, and the snake disappeared into the depths.
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