In Norse mythology, what demon is a giant serpent that encircles Midgard?

In Norse mythology, what demon is a giant serpent that encircles Midgard? The answer is Jörmungandr.

Also called the Midgard Serpent or World Serpent, Jörmungandr lives in the ocean surrounding Midgard, the human world. According to Snorri Sturluson’s Prose Edda, the serpent is a child of Loki and the giantess Angrboða, alongside Fenrir and Hel.

Odin cast Jörmungandr into the sea, where it grew so enormous that it could encircle the world and bite its own tail. This imagery resembles an ouroboros, a serpent forming a circle by swallowing its tail.

Jörmungandr is especially associated with Thor. In one famous episode, Thor nearly pulls the serpent from the sea while fishing. At Ragnarök, the two finally fight; Thor kills Jörmungandr but dies after only nine steps from the serpent’s venom. Fenrir is the giant wolf, Hel rules the dead, and Surtr is a fire giant—not the world-encircling serpent.