Thiassi was a frost-giant. Thiassi was the eldest son of Olvaldi or Allvaldi, and the brother of Idi and Gang. His father was very rich and lived in Thrymheim. At his death, the land was divided between the three brothers.
Thiassi had the ability to turn himself into an eagle. Thiassi had a beautiful daughter named Skadi.
Thiassi was the giant who had abducted Idun, keeper of the apples of youth, with the help of Loki. Loki was later forced to rescue Idun. Thiassi pursued Loki to Asgard in a form of a giant eagle, but was killed by the Aesir, as he passed over the wall of Asgard.
Skadi would have avenged her father, had the Aesir not made peace with her, by offering to her an Aesir husband. Odin had also taken Thiassi’s eyes and threw them in the sky, and two new stars were created.