Introduction
Jormungandr, the World Serpent or Midgard Serpent, is a son of Loki and the giantess Angrboda, cast by Odin into the great ocean surrounding Midgard, where he grew to such an immense size that he now encircles the entire world, biting his own tail. He is Thor's great enemy, and their mutual, final confrontation at Ragnarok is fated to kill them both.
This article follows Old Norse mythological tradition, chiefly the Poetic Edda and Snorri Sturluson's Prose Edda, noting where the two sources differ.
Family
Cast into the sea
On learning of prophecies concerning the danger posed by Angrboda's three children, Odin took action against each of them; Jormungandr he threw into the ocean that surrounds Midgard. There, the serpent grew so enormous that he now stretches all the way around the world, gripping his own tail in his mouth, giving him the alternate name Ouroboros-like epithet "Midgard Serpent".
Thor's fishing expedition
In one of the most vividly told episodes of Norse mythology, Thor, disguised as a young man and staying with the giant Hymir, goes fishing using an ox's head as bait and hooks Jormungandr from the depths. Thor is on the verge of killing the serpent with his hammer when the terrified Hymir, in most versions, cuts the fishing line, allowing Jormungandr to sink back into the sea, narrowly postponing their final confrontation.
Ragnarok
At Ragnarok, Jormungandr rises from the sea, and he and Thor finally meet in their fated final battle: Thor kills the serpent with Mjolnir, but, poisoned by its venom, manages only nine steps before collapsing dead himself.