Jormungand

(Also Jormundgand, Jormungandr, Midgardsorm)

"The Midgard Serpent"
"The World Serpent"

Jormungand was a monster in Norse Mythology. The middle-child of Loki and the giantess Angraboda (his siblings were Fenrir and Surt or Hel), he took the form of a giant serpent. The gods were frightened that Jormungand (who was growing in power) would cause strife among men and the deities, so Odin sent a group of gods to kidnap the serpent. Once they captured him, Odin threw him into the sea which surrounded the entire world, and there the snake grew and grew until he surrounded the entire world and could bite his own tail. The unending nature of Jormungand presumably represented eternity, as well as the fact that the serpent had the entire world in its coils.

At Ragnarok, the serpent will escape from the sea to dry land, and will spew poisonous venom all over the earth and sky. After assisting his brothers in the attempted destruction of the entire world, Jormungand will fight Thor. Thor will kill the serpent, but only after he has been poisoned by venom. He will take only nine steps after killing the snake before he dies.

The fight between the Midgard Serpent and Thor during Ragnarok is prefaced by the two mythical figures meeting three times. At their first meeting, Thor, Loki and two human servants traveled to Jötunheim and the hall of Utgard. At the hall, the giant Utgard tested the travelers' strengths. In one of these tests, the giant asked Thor to pick up his cat, but the god was only able to lift the cat's foot off of the ground before conceding defeat. It was later revealed that the cat was actually Jormungand, and the test was only a trick.

At their second meeting, Tyr and Thor traveled to the giant Hymir's hall. They were there to pick up a large cauldron with which to brew ale for the other gods. During the visit, Hymir and Thor went fishing, and Thor killed the giant's prize bull to use the head as bait. Under the sea, the Midgard Serpent took the bait and the god dragged the snake up out of the water, where he towered over the boat. Thor grabbed his hammer Mjollnir and threw it at Jormungand's head, but Hymir cut the line (he was afraid) allowing the serpent to sink back into the water before the hammer hit.

Their last meeting occurs at Ragnarok, where Jormungand has joined with Loki, Fenrir, Hel and an army of the dead. It is at the third meeting that Thor and the serpent kill each other.

Sources:
http://webhome.idirect.com/~donlong/monsters/Html/Midgarse.htm
http://www.pantheon.org/articles/j/jormungand.html

Jormungand is the old Norse name for the Midgard Serpent, a monster of ancient Norse mythology. Rare among monsters in that it supposedly lived in Midgard, the realm of Humans (most monsters of Norse mythology dwelled in Udgard, along with the evil giants who plotted against Gods and men alike). Jormungand was a gigantic serpent, which dwelled in the great sea that the Vikings believed surrounded all of the human world. The enormous beast encircled all of Midgard, so a careless sea explorer would certainly meet the Serpent if he ventured too far from land. Needless to say, an encounter with the Serpent was supposedly not a Good Thing at all.

Jormungand was the second child of Loki, the God of Deception, and a giantess named Angraboda. Considering that the two others were Fenris (a wolf monster) and Hel (who would later become the ruler of the Underworld), and that Loki was also the father (or, rather, mother -- he had assumed the form of a mare at the time) of Sleipnir, Odin's eight-legged horse, you could imagine that Loki's family reunions would be interesting settings, to say the least.

Thor, the Norse god of thunder, wasn't terribly fond of Jormungand. As myth would have it, whenever fishermen, sea explorers or Viking raiding parties encountered the Serpent, Thor would often come along to fight the great snake. It is possible that this particular myth arose because of thunderstorms at sea (the Norsemen believed that thunderstorms were caused by Thor riding across the sky in his goat-pulled chariot), where the ocean's waves ripped at ships like they were a gigantic snake. Ancient Norsemen spent a considerable amount of their time at sea, so many of their myths and legends revolve around such matters.

Thor attempted to kill Jormangund on several occasions, the most often-cited being an ill-fated fishing trip conducted by him and the frost giant Hymir. The two set out to catch the serpent, using an ox head for bait. The serpent took the bait, but the resulting struggle was too much for Hymir, who feared for his life and cut the fishing line. Jormungand escaped, angrier than ever.

During Ragnarok, Norse mythology's Armageddon, Jormungand rises from the oceans to participate in the final battle between good and evil, Gods and Giants. You can probably guess who it seeks out to fight. After an epic battle Thor finally kills the serpent, but dies shortly afterwards from Jormungand's venom.