Wulverblade draws a certain degree of inspiration from arcade and Genesis/Mega Drive classic Golden Axe, right up to including a literal golden axe that you can find. It’s insanely powerful, though sadly breakable like all strong attack weapons. Since I got the achievement for finding it and keep getting messaged about its location, I figure no one has pointed out where it actually is yet. It turns out that it’s hidden in one of those surprisingly obvious ways; in the fourth stage, there’s a cliff edge that you jump off of. Rather than jumping off right away, you can jump onto a second, lower path that wraps back around into a cave, and the axe is inside there.
Here’s a video of its location and demonstration of its power: