Night's Edge

The Night's Edge is a high-end melee weapon, crafted at a Demon Altar. It requires four of the currently most powerful sword-class weapons to craft: a Light's Bane, a Muramasa, a Blade of Grass, and a Fiery Greatsword. Although the damage in the tooltip is 40, the actual damage against monsters seems to be 45.

Putting any of the above four weapons in the Guide's "Crafting" option will reveal the full recipe and Demon Altar requirement.