The podcast "American Carnage" is a great journey if you don't know anything about John Brown. They mention this sword in the podcast, it's got a hollow compartment in it that contains liquid mercury. The mercury flows forward when it's swung in a slash, changing the weight distribution of the blade to kill the enemies of abolition with greater prejudice. Based
Edit: My only source for this was the podcast I mentioned, so take it with a grain of salt. I hope it's true though. It's such an interesting idea for a sword.
I hope they took the mercury out or the chamber has some non-metal finish, because mercury is one of those metals that gets absorbed into other metals making them brittle and weak and ultimately destroying the object.