Volcano

Only the first enemy you touch in a given swing sets off the explosion, and that blast catches a couple of neighbors around it. So wading into a wall of enemies does not give you a wall of explosions. The weapon is slow and heavy, and it pays off when a few things are bunched tightly rather than when a room is full.

The explosion always sets what it touches alight, which keeps damage ticking through the long, awkward pause before your next swing lands. On a slow sword that matters more than it would on something fast, since the ignite is doing work during time you would otherwise be contributing nothing.

It glows in the dark while held and leaves a trail of sparks when swung, so digging toward the Underworld with it out saves you a torch and a hand. Keep it after you stop using it in fights: it goes into the Night's Edge, which is the reason most players craft one at all.

It spent most of the game's life called the Fiery Greatsword. The Steam achievement art still shows it under that name, which is why the badge and the item do not match.