Once a year Odin must undertake the Odinsleep to regain his strength. During this time Asgard is vulnerable to attack from its many enemies, most notably Odin's adopted son, Loki. Loki first takes command of Asgard during the Odinsleep, using his right as the 'son' of Odin before Thor could claim it, but fled when Asgard was invaded by Mangog as he realized that this new foe was too powerful. Loki later usurped the throne of Asgard by taking the Odinring, but fled again when Asgard was invaded by the fire demon, Surtur.