Blue Moon

Blue Moons are Flails that can be found in dungeons. When used, it will emit light around it. This flail is stronger than the Ball 'O Hurt but weaker than the Sunfury.

This video shows the Blue Moon in action against the Eater of Worlds. It's particularly useful against this boss (as well as bone serpents) because it can go through them and hit all of their parts at the same time: