Jörmungandr is a sea serpent, the middle child of the giantess Angrboða and Loki. According to the Prose Edda, Jörmungandr was tossed into the great ocean that encircles Midgard. Instead of drowning, he simply grew so large that he was able to surround the earth and grasp his own tail, like an ouroboros. As a result, he received the name of the Midgard Serpent (Midgardsormr) or World Serpent. It is believed that when he lets go of his own tail, the world will end. Jörmungandr's arch-enemy is the god Thor.

The last meeting between the serpent and Thor is predicted to occur at Ragnarök, when Jörmungandr will come out of the ocean, and flood the world, or poison the sky. Thor will kill Jörmungandr but will die himself because of the serpent's venom.