Sunfury

The Sunfury is a rare drop flail category weapon from Bone Serpents in The Underworld. It's an orange flail that emits light during attack and deals 40 points of damage, but can hit multiple enemies at once and one enemy twice (or multiple times in the case of a boss). It has a 1/100 chance to drop from Bone Serpent.

When the ball goes too far from the player or hits a block, it quickly retracts. Like boomerangs and other flails, throwing the weapon behind a line of enemies will force it to retract through them all, dealing damage to every one of them. This is especially effective in dealing with Goblins, clumps of enemies, and large enemies.

The Sunfury is probably the best weapon in the game for defeating bosses. High damage is effective against armor, and flail-type weapons tend to hit large enemies multiple times per attack. The Eye of Cthulhu, for example, can be defeated without using a single potion by a moderately competent player.

Below is a video looking at the Sunfury:

How to farm a sunfury: