Courtesy of Ask ... A ... Nin ... Ja!
The Whichblade is a legendary weapon. The only problem is, you never know what you'll get when you draw it. It could be a rapier, could be the corkscrew from a Swiss Army knife, it could be Marv Wolfman, etc.
There is also the extremely, extremely broad sword.
And the Sai of Sighs. One nick and all you can do is exhale.
The Sai of Sighs would be a cool sword or dagger called Last Breath. It literally prevents you from inhaling after you take damage, so anything that requires breathing will suffocate if the initial wound is not enough to kill it. The legend surrounding the blade simply distills it into "anyone wounded by this blade has breathed his last breath." It is both literally (cannot inhale) and figuratively (is now dead) true.