Jörmungandr, sometimes referred to as the Midgard Serpent (or World Serpent) is a sea serpent of the Norse mythology, and the child of Loki and the giantess Angrboða. Its siblings are Fenrir, a monstrous wolf, and Hel, a female being presiding over the similarly named realm of the dead in Niflheim.
According to legend, Odin took Loki's three children, Fenrir , Hel and Jörmungandr, and tossed Jörmungandr into the great ocean that encircles Midgard. The serpent grew so large that he was able to surround the Earth and grasp his own tail. It was said that when he releases , the world will end. It is one of several events that herald the coming of Ragnarök. As a result, he received the name of the Midgard Serpent or World Serpent.
Jörmungandr's arch-enemy is the god of thunder Thor and myths have preserved the stories of at least three instances in which the two meet. The first is where Thor encounters the serpent in the form of a colossal cat, disguised by the magic of the giant king Útgarða-Loki, who challenges the god to lift the cat as a test of strength. Thor is unable to lift such a monstrous creature as Jörmungandr, but does manage to raise it far enough that it lets go of the ground with one of its four feet. When Útgarða-Loki later explains his deception, he describes Thor's lifting of the cat as an impressive deed.
The second is during Thor's fishing trip with the giant Hymir. After refusing to provide Thor with bait, Thor kills Hymir's ox and uses its head as bait instead. Thor then prepares a strong line and a large hook and baits it with the ox head, which Jörmungandr bites. Thor pulls the serpent from the water, and the two face one another, Jörmungandr dribbling poison and blood. Hymir goes pale with fear, and as Thor grabs his hammer to kill the serpent, the giant cuts the line, leaving the serpent to sink beneath the waves.
The last meeting between the serpent and Thor is predicted to occur at Ragnarök, when Jörmungandr will come out of the ocean and poison the sky. Thor will kill Jörmungandr and then walk nine paces before falling dead, having been poisoned by the serpent's venom.