Utgard-Loki is monarch of the Frost Giants, who for millennia have been the enemies of the immortal gods of Asgard. The Frost Giants dwell in Jotunheim, a realm of ice, snow, and cold, which lies in a dimension other than that of Earth. Jotunheim is one of the "Nine Worlds" of Asgardian cosmology. Utgard-Loki defeated Thor, prince of Asgard, in an encounter as yet unrecorded outside the Norse myths. More recently, Utgard-Loki learned of the disappearance of Odin, monarch of Asgard, and planned to take this opportunity to conquer Asgard. As a first step, Utgard-Loki and his Frost Giants invaded Nornheim and captured its ruler, the sorceress Karnilla, a sometime ally of Asgard. Utgard-Loki used his enchanted dust to turn the other inhabitants of Nornheim to stone, and forced Karnilla, who was prevented from using her powers by magic shackles, to work as a slave in his fortress. Thus Utgard-Loki deprived Asgard of a potential ally, and he planned to eliminate Asgard's other allies as well until Asgard would be forced to stand alone against invasion by Jotunheim. Karnilla's lover, Balder the Brave, one of Asgard's leading warriors, went to Jotunheim to rescue her.
Using his newly developed power to radiate light and heat, Balder melted the fortress of Utgard-Loki, which had been constructed from ice, and reduced Utgard-Loki and his fellow giants to tiny size. Balder freed Karnilla, took her back to Nornheim, and then returned to Asgard, where soon afterward, he was made Odin's successor as monarch of Asgard. But whether or not Utgard-Loki will remain in greatly reduced size remains to be seen, and the Frost Giants will soon launch a new attack against Asgard.
Like all the giants of Jotunheim, Utgard-Loki, needs cold temperatures to maintain his existence. Great heat will cause the Frost Giants to "melt," thereby shrinking in size. Continued exposure to such heat would case a Frost Giant to liquefy completely.