Tsortean Knot

Carelinus, the greatest conquerer in the history of the Disc was famous for, amongst other things, untying the Tsortean Knot by cutting through it with a sharp sword. At least, he was according to the Minstrel that Cohen the Barbarian forced to accompany the Silver Horde to Dunmanifestin.

This may be considered cheating by some, but as Cohen pointed out upon being told the story, it was a really big sword, and there was this great army right outside the door, and maybe telling him that wouldn't be the best idea...

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On Roundworld, it was Alexander the Great who untied the Gordian Knot in the same way.