Broken Hero Sword

Mothron is the only source, and Mothron only turns up during a Solar Eclipse once Plantera is down. The drop is not reliable, so an eclipse can hand you several or none at all. Spawn count is the part you control, so bring a Water Candle and a Battle Potion and fight somewhere the eclipse enemies can actually reach you. More Mothrons per eclipse is the whole plan.

The drop separately checks that all three mechanical bosses are dead, on top of Mothron appearing at all. In a normal run you will never see this, since you cannot get to Plantera without them. It only shows up when something outside the game puts Mothron in front of you early, at which point it will refuse to hand over the sword.

It leans on an old story beat: the shattered blade of a hero who lost, recovered and reforged by whoever comes next. Given what you take it from, the thing that broke it is standing right there.