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So its basically the new Giants Knife.

I'm pretty sure this is Nintendo doing another clever 'take that' at how exquisitely any "imposing/ultra powerful/whatever" weapon names can be peppered with hyperbole.

Really every weapon in this game breaks pretty quickly, at least early game, so the eightfold blade isn't really any different from other weapons. Although it is a katana so it makes the jokes about how katanas were actually pretty fragile even easier.

I haven't found it yet, what was it like? Did it just last for three swings like a tree branch?

It's alright damage wise, but it's a bit low durability for being a mid-tier weapon. Surprisingly, it's actually a slow 2-handed weapon even though katanas are always portrayed as being fast 1-handed weapons, so bonus points to Nintendo for getting that part right.
I think I found it at one of the 2 dueling peaks shrines if you want to try it out yourself.

I found one, took out an enemy camp with it. It actually had durability up, but it still didn't last.