The stoor worm, or Mester Stoor Worm, was a gigantic evil sea serpent of Orcadian folklore, capable of contaminating plants and destroying animals and humans with its putrid breath. It is probably an Orkney variant of the Norse Jörmungandr, also known as the Midgard Serpent, or world serpent, and has been described as a sea dragon.
The king of one country threatened by the beast's arrival was advised to offer it a weekly sacrifice of seven virgins. In desperation, the king eventually issued a proclamation offering his kingdom, his daughter's hand in marriage, and a magic sword to anyone who could destroy the monster. Assipattle, the youngest son of a local farmer, defeated the creature; as it died its teeth fell out to become the islands of Orkney, Shetland and the Faroes, and its body became Iceland.
Similarities between Assipatle's defeat of the monster and other dragon-slayer tales, including Herakles' destruction of a sea monster to save Hesione, have been noted by several authors. It has been suggested that tales of this genre evolved during a period of enlightenment when human sacrifices to bestial divinities were beginning to be suppressed.