A situation in which one appears to have narrowly escaped disaster, only to suffer a fatal blow at the last moment. Refer's to the weapon of Durin's Bane who, after falling into the chasm below Khazad-dûm, lashed out its whip and caught Gandalf by the heel.
1

"Dude, why'd you two break up? I thought you were fine."
"The Balrog's whip."
"Huh?"
"I asked if she was okay, and she was like, ' yeah – I just think it's funny how...'"

2 I stared into the bowl, my turd swirling into the abyss. But, at the last moment, the Balrog's Whip struck, and its girth jammed in the waste pipe. In the hall, someone waited for their turn, ignorant of this dreadful news.
by Leery Loon February 4, 2021
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