Sunfury

The Sunfury is a rare drop flail category weapon from Bone Serpents in The Underworld, or as a very rare drop from Skeletron. 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.

Video displaying the weapon:

Best method to obtain
An easy way to get the Sunfury is to build a platform about 20 blocks long out of hellstone (because it is dynamite proof) with side walls going up about 10 blocks on either side. Then build a wood platform going all the way across the top. Then when a Bone Serpent spawns, as soon as it's head goes below the hellstone platform throw a dyamite stick down. Then jump up to your wood platform and stand directly above the dynamite. If timed correctly, the Bone Serpent will fly back up towards you as the dynamite explodes directly in the centre of the bone serpent. The reason this is an easy way to get it is because each section of a bone serpent has a 1/100 chance of dropping a Sunfury/Flamelash. This means that you are killing about 10+ sections of the bone serpent at once instead of just one section.