Jörmungandr battled the Æsir many times, and Thor in particular came to be his hated enemy. The two would clash a total of three times.
Thor encountered the World Serpent for the first time while traveling in Jotunheim, the land of the giants. The king of the giants, Utgarda-Loki, asked Thor to prove his strength by lifting the king’s cat. Absolutely confident in his ability to perform such a simple task, Thor accepted the challenge. To his consternation, however, he couldn’t budge the cat. Even after bringing his full immortal strength to bear, he was forced to admit defeat having only managed to lift one of the cat’s paws off the ground. Confronted by the humiliated and enraged Thor, Utgarda-Loki confessed that the creature Thor had managed to partially lift was in fact Jörmungandr, disguised by magic.
The second ecounter occurred when Thor went fishing with the giant Hymir. Ignoring Hymir’s warnings, Thor rowed out into the open sea and cast his line, using the head of an ox as bait. Sure enough, Jörmungandr latched on. After a brief struggle, Thor managed to raise him above the waters and prepared to slay the great snake. He was evidently still miffed about the cat incident. Fearing the World Serpent, Hymir finally intervened and cut the line before Thor’s blow could connect, and Jörmungandr sank back into the depths.
The third and final meeting between Thor and Jörmungandr is fated to occur at the apocalyptic battle of Ragnarök, where the Æsir and their enemies will clash for the final time. Thor succeeds in killing Jömungandr with his legendary warhammer Mjölnir, but not before being sprayed by the serpent’s venom. He manages to take nine staggering steps before succumbing to the poison and falling to the ground, dead.