Vápnthjófr saga

Loki had three children together with the jotun Angerboda- The fenrir wolf, the midgard serpent and Hel. These three were foretold to bring about Ragnarök, and when the other gods found out about these children’s existance, they got rid of them- The fenrir wolf was bound with a magical unbreakable chain, the Midgard serpent was sealed below the ocean and Hel was cast into Helheim.