According to Asgardian legend, in the beginning there was nothing, but in time two worlds came into being on opposite sides of the void. The one to the north was named Niflheim, a world of clouds and shadows in whose center surged the fountain Hvergelmir, from which flowed twelve rivers of ice. The one to the south was named Muspelheim, which teemed with rivers of fire. Eventually the warm air from the south carved out the frost giant Ymir from the ice in the north. Ymir became the father of all the giants, and his cow Auðumbla licked out of the ice the first Asgardian, Buri. Buri had a son named Borr, who married the giantess Bestla. Borr and Bestla had three sons named ****, Vili, and Ve, who were known as the Æsir. **** and his brothers grew to hate the giants and slew Ymir, and his blood formed a great sea. **** and his brothers then raised Ymir's body from the sea and created Midgard between Niflheim and Muspelheim. With Ymir's bones they created mountains, and with his hair they created trees. They then raised Ymir's skull upon four pillars to create the heavens. Within the skull contained sparks from Muspelheim, which became the sun, moon, and stars. When Midgard was complete, **** and his brothers created a home for themselves above it called Asgard. Between the two worlds they stretched a rainbow bridge and called it Bifröst.Once a year **** must undertake the Odinsleep to regain his strength. During this time Asgard is vulnerable to attack from its many enemies, most notably ****'s adopted son, Loki. Loki first takes command of Asgard during the Odinsleep, using his right as the 'son' of **** 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.