Mechanical Wheel Piece

Expert worlds only, and only out of the Twins bag, with the matching parts sitting in the other two mechanical boss bags. The kit they build toward cannot be assembled until all three are dead, so a lone piece in your chest after one fight is not a sign you missed a step.

Do not sell it. Nothing else in the game wants it, and resummoning a mechanical boss to replace one component is a slow way to fix a mistake you never needed to make.