The Muramasa is a blue broadsword that is found in the Dungeon's Locked Gold Chests or in Golden Lock Boxes, both of which are opened via a Golden Key.

Can you craft the Muramasa in Terraria?

The Muramasa can be swung continuously by holding down the attack key. It also emits a faint amount of blue light upon being swung, much like a Blue Torch, the Hammush, and the Mushroom Spear when being swung. This sword cannot be crafted and can only be found inside the Dungeon. It emits a humming sound when swung.

How do you get the Murasama in Terraria?

It can be found in The Underworld's Bio-center Arsenal Lab, contained within a Security Chest. Although Murasama is technically attainable in Pre-Hardmode, it cannot be swung until Jungle Dragon, Yharon has been defeated in the current world. It possesses a larger than normal critical strike chance.