Jotunheim is one of the Nine Realms and the home of the Frost Giants. The planet is seen to be deteriorating from the lack of ice magic that comes from the Casket of Ancient Winters. It was ruled by Laufey.
Battle of Jotunheim
In 965 A.D., Jotunheim was the setting of the final battle between the Frost Giants and the Asgardians during the Asgard-Jotunheim War. After the battle, Odin took the Casket of Ancient Winters, depriving the Frost Giants of their power, and Laufey's son, who was left to die by his own father.
Without the Casket, the realm began to decay and break apart so that the full force of the Bifrost could rip it apart.