Jörmungandr
In Norse mythology, Jörmungandr (Old Norse: Jǫrmungandr, lit. 'the Vast 'gand'', see Etymology), also known as the Midgard Serpent or World Serpent (Old Norse: Miðgarðsormr), is an unfathomably large sea serpent or worm who dwells in the world sea, encircling the Earth (Midgard) and biting his own tail, an example of an ouroboros. As a result of it surrounding Midgard (the Earth) it is referred to as the World Serpent. Jörmungandr releasing its tail is one of the signs of the beginning of Ragnarök (the final battle of the world).
Jörmungandr is said to be the middle child of the trickster god Loki and the giantess Angrboða. According to the Prose Edda, Odin took Loki's three children by Angrboða – the wolf Fenrir, the goddess Hel, and the serpent Jörmungandr – and removed them from Asgard (the world of the Æsir). The serpent Jörmungandr was tossed into the great ocean that encircles Midgard. There the serpent grew so large that it was able to surround the Earth and grasp its own tail. The old Norse thunder-god, Thor, has an on-going feud with Jörmungandr in their epics and the two can be seen as archfoes. During Ragnarök, Thor and Jörmungandr will fight each other to the death.