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[The Saucepan Man]

what was the name Gandalf gives the demon he slays atop mountains of moria

When he confronts the Balrog on the bridge of Khazad-Dum, Gandalf calls it "Flame of Udûn". Udûn is Sinarin for "The Pit" or "The Underworld", from the Quenya "Utumno", ie the first Realm of Morgoth. So, it was a reference based upon the Balrog's form and history, rather than a name.