Idun was the guardian of the magic apples of youth, the apples that kept the gods young. Her husband was Bragi, the God of Poetry.
When the frost giant Thiassi captured the trickster Loki, he promised Loki's release only if Loki would bring him the apples of youth. Loki tricked the trusting Idun into accompanying him into the forest where he had supposedly seen apples of better quality. Thiassi, in the guise of an eagle, took Idun and the apples in his claws and flew to Jotunheim, the land of the frost giants.
The loss of the apples caused the gods to become weak in body and mind and they even started to fear death. Odhinn threatened Loki with magic and Loki went to Jotunheim in the form of a falcon. He turned Idun into a nut and fled back to Asgard with the frost giant in hot pursuit as an eagle. However, the inhabitants of Asgard had lit fires on the walls and Thiassi burnt to death. Idun returned to her female form and returned her apples to the gods.