Muspelheim is a dyson sphere and one of the Nine Realms.
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During the Convergence of 2013, Muspelheim was one of the realms that could be seen from Earth during the Battle of Greenwich. As the Convergence went underway, Malekith attempted to use the power of the Aether to turn Muspelheim and all the other Nine Realms into complete darkness, until he was stopped and killed by Thor before the end of the Convergence.
Two years after the Battle of Sokovia, Thor has been unsuccessful in searching for the Infinity Stones, and is now imprisoned by the Fire Demon Surtur on Muspelheim. Surtur reveals that Thor's father Odin is no longer on Asgard, and that the realm will soon be destroyed by Ragnarök, once Surtur unites his crown with the Eternal Flame that burns beneath the city. Thor clashes with and defeats Surtur, taking his crown, believing he has prevented Ragnarök. This leads to a lengthy battle on Muspelheim. During the battle, Surtur's Fire Demons stormed on the Asgardian who used the Mjolnir to defeat them all. In the process, the Fire Dragon broke free and chased after Thor, who managed to escape to Asgard with the dragon's severed head, thanks to the help of Skurge.
- In the non-canon video game Thor: God of Thunder, Muspelheim was the home of the Fire Demons, and considered to be the most dangerous of all, its hellish environment being hazardous even to Asgardians.
- In Norse Mythology, Muspelheim is the primordial realm of fire, light, and heat whose flames combined with the rime flows of Niflheim to form water which the gods used to create all of Midgard.