Proper noun
Muspell
(Norse mythology and Germanic paganism) The primordial realm of fire which existed to the south of Ginnungagap.
High notes that, although the bridge is strong, it will break when " Muspell 's lads" attempt to cross it, and their horses will have to make do with swimming over "great rivers." Source: Internet
When the "sons of Muspell" ride over Bifröst it will break, "as was said above." Source: Internet