The world serpent coiled around the realm of humans
The great serpent born to Loki and Angrboða. Odin cast it into the deep sea that girdles Midgard, the realm of humans, but it grew large enough to coil around the whole world and bite its own tail, and so is named the "Midgard Serpent." It and the thunder god Thor are fated foes — Thor once hooked it and let it back into the sea, and at Ragnarök the two will at last meet head-on and slay each other.
At Ragnarök, Jörmungandr surges ashore, spewing venom that blackens sea and sky. Thor advances to meet it and, swinging Mjölnir, beats the serpent dead. But its venom has already soaked into his flesh — Thor staggers back nine steps and falls dead. Man and serpent, their old feud settled, go down to death together.
Below are how modern works borrow or reinterpret this name or story — not the original material. The two differ, so keep them apart.