Myths of the Norsemen From the Eddas and Sagas
Page: 55The seeming eagle, who was the storm giant Thiassi, at last agreed to release Loki upon one condition. He made him promise upon the most solemn of oaths that he would lure Idun out of Asgard, so that Thiassi might obtain possession of her and of her magic fruit.
Released at last, Loki returned to Odin and Hoenir, to whom, however, he was very careful not to confide the condition upon which he had obtained his freedom; and when they had returned to Asgard he began to plan how he might entice Idun outside of the gods’ abode. A few days later, Bragi being absent on one of his minstrel journeys, Loki sought Idun in the groves of Brunnaker, where she had taken up her abode, and by artfully describing some apples which grew at a short distance, and which he mendaciously declared were exactly like hers, he lured her away from Asgard with a crystal dish full of fruit, which she intended to compare with that which he extolled. No sooner had Idun left Asgard, however, than the deceiver Loki forsook her, and ere she could return to the shelter of the heavenly abode the storm giant Thiassi swept down from the north on his eagle wings, and catching her up in his cruel talons, he bore her swiftly away to his barren and desolate home of Thrym-heim.
“Thrymheim the sixth is named,
Where Thiassi dwelt,
That all-powerful Jötun.”
Lay of Grimnir (Thorpe’s tr.).
Isolated from her beloved companions, Idun pined, grew pale and sad, but persistently refused to give Thiassi the smallest bite of her magic fruit, which, as he well knew, would make him beautiful and renew his strength and youth. 
“All woes that fall
On Odin’s hall
Can be traced to Loki base.
From out Valhalla’s portal
’Twas he who pure Iduna lured,—
Whose casket fair
Held apples rare
That render gods immortal,—
And in Thiassi’s tower immured.”
Valhalla (J. C. Jones).
Time passed. The gods, thinking that Idun had accompanied her husband and would soon return, at first paid no heed to her departure, but little by little the beneficent effect of the last feast of apples passed away. They began to feel the approach of old age, and saw their youth and beauty disappear; so, becoming alarmed, they began to search for the missing goddess.
Close investigation revealed the fact that she had last been seen in Loki’s company, and when Odin sternly called him to account, he was forced to admit that he had betrayed her into the storm-giant’s power.
“By his mocking, scornful mien,
Soon in Valhal it was seen
’Twas the traitor Loki’s art
Which had led Idun apart
To gloomy tower
And Jotun power.”
Valhalla (J. C. Jones).
The Return of Idun
The attitude of the gods now became very menacing, and it was clear to Loki that if he did not devise means to restore the goddess, and that soon, his life would be in considerable danger.