Idun is one of the most prominent goddesses in Norse mythology. She’s the keeper of mysterious fruits eaten by the gods, which enable them to retain their youth and ward off the process of aging. These fruits are commonly assumed to be apples, but, as Old Norse scholar E.O.G. Turville-Petre has pointed out, the word used to describe them, epli, was applied to all sorts of fruits and nuts. Proper apples were unknown in Scandinavia prior to the arrival of Christian civilization, so we may assume that, in the original tale, epli referred to some form of berries or nuts.
What happens when the gods lose Idun and her life-preserving medicine?
The Kidnapping of Idun
Three of the Aesir gods, Odin, Loki, and Hoenir (pronounced “HIGH-neer”), were on a journey that took them through desolate mountains far from Asgard. Food was scarce in this uninviting region, so when the travelers came upon a herd of oxen, they slaughtered one for their dinner.
When they put the meat over their fire, however, it didn’t cook, no matter how long they left it there. Perplexed by this, they heard a voice addressing them from above. Looking up, they saw a very large eagle perched on a nearby branch. “It is I,” he said, “who, by my magic, prevent your catch from cooking. But if you will give me my fill of the meat, then I shall release the remainder from my spell.” The gods, though irritated, agreed, and the eagle flew down and took for himself the choicest portions of the ox.
Loki thought this to be beyond the terms of their bargain, and, in anger, took up a sturdy branch and lunged at the eagle. The eagle snatched the branch in his talons, and, with a bewildered Loki still clinging to the other end, flew up high into the sky. The terrified god begged the eagle to release him, but the eagle – who was none other than Thjazi (pronounced “THYAH-zee”) the giant in disguise – refused to do so until Loki swore an oath to bring him Idun and her fruits.
When the trio made it back to Asgard at the conclusion of their travels, Loki went to Idun and told her that he had found fruits even more marvelous than her own growing in a forest beyond the walls of Asgard, and that she should follow him there and bring her own apples for comparison. Idun followed the trickster, and when she reached the wood she was borne up by Thjazi in his eagle form, and taken away to the giant’s abode. This place was called Thrymheim (“Thunder-Home”), and was situated in the highest mountain peaks, whose icy towers growled down at the fertile fields below.
In Idun’s absence, the gods and goddesses felt old age creeping up on them. Their skin became wrinkled, their hair greyed, and their vigor waned. When they assembled together and asked one another about the circumstances under which Idun was last seen, it was reported that the last sighting of her had been with Loki as the two left Asgard together. Then they seized Loki and threatened him will all manner of pains if he didn’t tell them what had happened to the fair goddess. Loki spilled his story, and the gods informed him that if he couldn’t rescue Idun from Thjazi, he would be put to death.
Freya lent him her hawk feathers, with which one can shift his or her shape into that of a hawk, and he flew off to Jotunheim, the homeland of the giants, within which Thrymheim was located. When he came to Thrymheim, he found, to his great delight, that Thjazi had gone out to sea to fish, leaving Idun home alone. Without losing a minute, Loki turned the goddess into a nut and sped away with her in his talons.
When Thjazi returned and found his prize missing, he assumed his eagle form and filled the air with the thunderous beats of his wings on his way toward Asgard in pursuit of Loki. By the time the god was in sight of his home, the giant was close behind him and furiously closing the gap. When Loki’s companions caught sight of the chase, they built a pile of kindling around their fortress. Loki, still clutching Idun, made it across the barrier. And then the gods lit the fire, and it exploded so rapidly that Thjazi didn’t have time to turn around before entering the flames. And that was the end of his flight.
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 Turville-Petre, E.O.G. 1964. Myth and Religion of the North: The Religion of Ancient Scandinavia. p. 186.
 Þjóðólfr. Haustlöng.
 Snorri Sturluson. The Prose Edda. Skáldskaparmál 2-3.