Ymir was formed at the beginning of creation in a place called Niffleheim. He is an ancient being of limited intelligence. The oldest and most powerful of the Frost Giants, he tends to focus all of his energies towards destruction and little else. This may in fact be Ymir's only weakness, and the reason why Odin and his two brothers, Ville and Ve, were able to first defeat him.
Except for Surtur, Ymir is the oldest known being in the "Nine Worlds" of the Asgardian realm. The Frost Giants origin is lost in antiquity; however, they are known to predate the gods of Asgard themselves. Few in number due to the heavy losses sustained over the millennia in battle with the gods, the Frost Giants dwell in the frozen land of Niffleheim north of the country of Asgard proper. Their huge size, durability, and raw power make them the gods most formidable foes.
Age has not diminished Ymir's power in the least. Although possessing some intelligence and capable of speech, Ymir is not particularly cunning or subtly motivated. His vast physical power has enabled him to survive and maintain his rule over his kinsmen over the millennia.
Ymir had once been imprisoned by Odin at the beginning of the age within a volcano. It is not known how long he remained there. He was once magically conjured to appear on Earth by the human cult of demon-worshipers called the Sons of Satannish, along with Surtur, via the spell of Fire and Ice. They planned to use Surtur to heat up the arctic climate and Ymir to cool off the African plains. They were dispatched from Earth when Doctor Strange used the Crystal of Conquest, which contained part of the mystical power of the demon known as Satannish and Mephisto, to transport them magically into each other's presence at the very instant they were lashing out with their weapons. They struck each others weapons canceling out each others powers and in turn were returned to their home dimension.
