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.

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