Blue Moon

Hold the button and it orbits you as a moving shield, hitting anything that wanders close without you aiming at all. Tap it and the head launches out and returns. The spin is the mode that matters at this stage, because it lets you handle a swarm while concentrating on where your feet are. Dungeon corridors are exactly the shape it wants.

Keep a Sunfury in your inventory and both flails deploy together, mirrored on either side of you. That covers both flanks with no extra input, which is a real upgrade when you are holding a doorway. Each one gives up some spin damage while they are paired, so it is not free, but two sides beats one whenever things are coming from both directions.

It comes out of the locked chests in the Dungeon, which means keys, which means grinding Dungeon enemies after Skeletron. Fishing up Dungeon Crates is the patient alternative. Neither route is quick, and that chest pool holds stronger weapons than this one, so treat it as something you use because it turned up rather than something you farm for.