The Blue Moon is a Flail that can be found in Dungeons. When used, it will emit light around it. The Blue Moon is stronger than the Ball O' Hurt but weaker than the Sunfury. When the Blue Moon is thrown far from the player or hits a block, it will quickly retract, throwing the weapon behind a line of enemies will force it to retract through them all, dealing damage to every one of them. There is another way to use the "retract method" as mentioned above. If you turn around when it's coming back to you, you will swing it the other way; you can do this an infinite number of times, as long as you time it correctly. It looks like you are spinning it around and around. The Blue Moon is especially effective in dealing with Goblins, groups of enemies and enemies with multiple sections, such as Eater of Worlds.