Magma Stone

Every melee hit sets the target burning, and it applies the harsher version of the burn rather than the basic one. It also paints fire across your weapon and its projectiles. That is decorative until you realize it emits light, at which point a yoyo with this equipped becomes a way to map a cavern by throwing it around corners ahead of you.

Anything the game counts as melee damage picks the effect up, including pickaxes, axes and hammers, so mining lights itself. Whips are the exception, which catches summoners out, since whips otherwise inherit most melee side effects.

Hellbats drop it rarely enough that camping them is miserable. Lava Bats are much better odds and they only appear once you are properly into Hardmode, so this tends to be something you pick up on a return trip rather than on your first descent.

It stacks with Frost armor and with flasks, so a melee build can carry several damage over time effects at once and let them run while it repositions. During the Wall of Flesh the boss itself shrugs the burn off, but The Hungry hanging from it do not, and clearing those is most of that fight anyway.