The Midgard Serpent - Jörmungandr

According to the Prose Edda, Odin took Loki's three children by Angrboða and tossed Jörmungandr into the great ocean that encircles Midgard. The serpent grew so large that it was able to surround the earth and grasp its own tail. As a result, it received the name of the Midgard Serpent or World Serpent. When it releases its tail, Ragnarök will begin. Jörmungandr's arch-enemy is the thunder-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 poison the sky. Thor will kill Jörmungandr and then walk nine paces before falling dead, having been poisoned by the serpent's venom.