Thiassi was a powerful giant that lived in Thrymheim, a mountainy area in Jotunheim. He had at least two brothers and he was the son of a rich giant called Alvaldi, the all-powerful. His daughter was Skadi, who would become one of the Aesir after her father’s passing.
He could shapeshift and appears as a giant eagle in one story, in which he kidnaps Idunn. He is killed when giving chase to Loki and Idunn when they are trying to return to Asgard.