The giant Thiazi is the personification of winter, he is a son of the giant Olvaldi. He is also a shapeshifter and the father of the giantess Skadi, wife of the Vanir god Njord. Thrymheim is Thiazi's mountain stronghold in Jotunheim, land of the giants. Like all giants, he is well versed in the use of magic.

With the aid of a magic eagle skin, he flew to Midgard where the gods Hoenir, Loki and Odin were cooking an ox. Still in the form of an eagle, he asked the gods for some of the meat. The gods agreed, but he ate so much that Loki became angry and impaled the eagle with a wooden staff. The eagle took flight with Loki still grasping the staff, unable to free himself. Thiazi released Loki on the condition that he would bring him the goddess Idun and her apples of youth.

Loki returned to Asgard and, a week later, went to the hall of Idun and told her that he had found a tree in Midgard that bore golden apples like hers. He offered to show her the tree and together they crossed the rainbow bridge Bifrost and entered Midgard. Idun carried her basket of apples as usual.

Still in the form of a giant eagle, Thiazi swooped down and carried Idun and her basket of golden apples to Thrymheim. Without the apples of youth the gods began to suffer the effects of old age. Loki soon fell under suspicion and admitted his role in the abduction of Idun. The gods then threatened him with death if he failed to return Idun and her apples to Asgard.

Borrowing a magic falcon skin from the goddess Freya, Loki assumed the form of a falcon, flew to Thrymheim, located Idun and transformed her into a nut. Then he took wing and began the long flight back to Asgard. When Thiazi returned to his stronghold he realized that Loki had taken Idun and the apples of youth. Taking the form of an eagle, Thiazi flew after Loki as fast as he could.

Thiazi was close behind Loki as the two birds neared the outer wall of Asgard. As soon as Loki flew over the wall, Odin ordered the gods to set fire to piles of wood and shavings. Thiazi's wings were badly burned, and he plunged to the ground where he was killed by the gods. Loki then returned Idun and her apples to their normal form. Odin transformed the giant's eyes into two stars.

Also known as: Thiassi, Thjatsi