Chain Knife

It is a flail with a very short leash. You get a bit of reach past a sword swing and a projectile you can steer, which in a tight corridor is genuinely useful, and in open ground is not. Treat it as a stopgap you use until something better drops, not a weapon you build around.

Cave Bats carry it, at odds low enough that finding one is luck rather than a farm. If you want it, mine normally and it shows up eventually.

On Remix and Zenith worlds it trades places with the KO Cannon. It drops from Clowns instead of bats, swings faster, reaches roughly twice as far, and hits hard enough to carry a fight. If you have been ignoring it on a normal world, that is why it looks so different when someone recommends it.

The Chain Guillotines are the same weapon done properly, so the flail playstyle is worth trying here before you commit to it later.