Jormungandr

Jörmungandr (Old Norse: Jörmungandr), also called the Midgard Serpent, is the son of Loki and Angrboða. Odin cast the serpent into the sea surrounding Midgard, where it grew so enormous it could bite its own tail and encircle the entire world — hence the name Midgard Serpent and the symbol of the Ouroboros. He is Thor's arch-enemy. They have met several times, including when Thor fished for Jörmungandr with the giant Hymir and nearly hauled him up. At Ragnarök they slay each other: Thor kills the serpent with Mjölnir but dies himself after taking nine steps — felled by its venom.