The Story of Fenris
Tyr was generally spoken of and represented as one-armed, just as Odin was called one-eyed. Various explanations are offered by different authorities; some claim that it was because he could give the victory only to one side; others, because a sword has but one blade. However this may be, the ancients preferred to account for the fact in the following way:
Loki married secretly at Jötun-heim the hideous giantess Angur-boda (anguish boding), who bore him three monstrous children—the wolf Fenris, Hel, the parti-coloured goddess of death, and Iörmungandr, a terrible serpent. He kept the existence of these monsters secret as long as he could; but they speedily grew so large that they could no longer remain confined in the cave where they had come to light. Odin, from his throne Hlidskialf, soon became aware of their existence, and also of the disquieting rapidity with which they increased in size. Fearful lest the monsters, when they had gained further strength, should invade Asgard and destroy the gods, Allfather determined to get rid of them, and striding off to Jötun-heim, he flung Hel into the depths of Nifl-heim, telling her she could reign over the nine dismal worlds of the dead. He then cast Iörmungandr into the sea, where he attained such immense proportions that at last he encircled the earth and could bite his own tail.
“Into mid-ocean’s dark depths hurled,
Grown with each day to giant size,
The serpent soon inclosed the world,
With tail in mouth, in circle-wise;
Held harmless still
By Odin’s will.”
Valhalla (J. C. Jones).