Exotic Scimitar

The sword rewards uninterrupted swinging. Land hits and it hits harder, stop for a moment and the bonus drains off fast, so it is at its best against a crowd or a boss you can stay glued to. Every dodge costs you some of it. There is a small buffer built in: the counter keeps climbing past the point where extra damage stops showing on screen, so a brief gap in your attacks eats the hidden surplus before it eats anything real.

The bonus rides on top of your melee bonuses rather than being folded in among them, which means it is worth more the better your gear already is. It also survives a weapon swap only if you swap to a Falcon Blade or a Cutlass, since those feed the same hidden counter. Anything else and you start from zero.

The Dye Trader has to die, and you cannot do it with your own weapons. That means arranging an accident: a trap, a fall, drowning, an explosive bunny. Lava works too, but keep the pool shallow or the sword burns up before you can pick it up.