Traditionally one of the Nine Worlds of Asgardian cosmos, one that with Niflheim, through their combination of extreme cold and heat created the other worlds in the Ginnungagab (Void) of primordial times.
Muspelheim is the lands of fire, considered a world of intense heat, featuring volcanoes and lava flows and such. In Marvel comics, as in Norse mythology, Muspelheim is the home of the fire jotunn or fire giants and the infamous fire giant; Surtur, who is destined to be the bane of Asgard.
In Marvel Comics version, as depicted in the Thor comics, Muspelheim was once visited by Odin and his brothers, who were out on an adventure together. While they visit, they encounter Surtur, who reveals his destiny to destroy Asgard through the combination of his mighty Sword of Twilight Doom and the Eternal Flame. The three brothers assault Surtur in order to prevent this, succeeding to break his sword at the cost of Odin's brothers lives, creating the phenomenon called the Odin Force.