Litho's Armory/LS-MOONLIGHT

The LS-MOONLIGHT is a hardmode broadsword that can be dropped from any of the biome-exclusive Mimics (Corrupt Mimic, Crimson Mimic, and Hallowed Mimic), and is a legendary weapon. Using the right mouse button fires an energy beam from it, and its base damage grows in strength with each vanilla hardmode boss defeated.

Trivia

 * This item, along with the WA-FINGER and KARASAWA-MK2, is a reference to the Armored Core series of video games.
 * The appearance of the LS-MOONLIGHT matches that of the part of the same name from Armored Core 2, and shares its description of "Compressed energy for amazing power".
 * The tooltip references the fact that a Moonlight Greatsword also appears in multiple Dark Souls video games. Both Dark Souls and Armored Core were developed by From Software.