Jörmungandr. Digital painting (presumably) by Alex Tornberg.
In Norse mythology, Jörmungandr (Old Norse: Jǫrmungandr, pronounced [ˈjɔrmuŋɡandr̥], meaning “huge monster”), often written as Jormungand, or Jörmungand and also known as the Midgard Serpent (Old Norse: Miðgarðsormr), or World Serpent, is a sea serpent, the middle child of the giantess Angrboða and Loki. According to the Prose Edda, Odin took Loki’s three children by Angrboða, the wolf Fenrir, Hel, and Jörmungandr, and tossed Jörmungandr into the great ocean that encircles Midgard. The serpent grew so large that he was able to surround the earth and grasp his own tail. As a result, he received the name of the Midgard Serpent or World Serpent. When he lets go, the world will end. Jörmungandr’s arch-enemy is the god Thor. It is an example of an ouroboros.
At the destruction of the universe, Jormungand and Thor will kill each other.