Muspelheim (also called Muspel) is the realm of fire, in Norse mythology. The fire giants live there. Their king is Surtr.
According to the Ragnarok prophecies, the Sons of Muspel will break the Bifrost bridge, signaling that time is ending:
||In the midst of this clash and din the heavens are rent in twain, and the sons of Muspel come riding through the opening. Surtr rides first, and before him and after him flames burning fire. He has a flaming sword, which shines brighter than the sun. In Ragnarok, Surtr kills Freyr with his flaming sword. As they ride over Bifrost it breaks to pieces, as has before been stated. The sons of Muspel direct their course to the plain which is called Vigrid.... The sons of Muspel have there effulgent bands alone by themselves.