Shadow Katana (5e Equipment)
From D&D Wiki
Weapon (longsword, Heavy), Rare
In some cultures this strong, single-edged sword is a symbol of honor and prestige. The katana is intended for two-handed use, but it's light enough to be wielded one-handed.
Strike from the Shadows. After you take an attack action, a shadow of you is left behind, that deals 1/4 of the damage dealt, rounded up.