Jormungand (“Great Wand” or the “Great Magic-Beast”), is the giant sea serpent and one of the offspring of Loki and Angerboda. The Aesir knew Jormungand would be dangerous for humanity, so they tossed him into the waters that encircled Midgard. Jormungand grew so big that he was able to encirlce the earth and bite his own tail.(see Ouroboros).
During Ragnarok, Thor will finally kill Jormungand, but not before Jormungand can spray poison on Thor and kill him as well.