Midgard Serpent

The great snake which lies in the ocean and encircles the world, its tail in its mouth. On the day of Ragnarok, the world will disappear under the ocean's waters when the Midgard Serpent rises from the sea. Thor will kill the Midgard Serpent but will be killed by the Serpent's poison.

Sources: Various

Norse/Teutonic Gods and Goddess
God and Goddess Index