This item appears to be a longsword hilt. While grasping the hilt, you can use a bonus action to cause a blade of pure darkness to spring into existence, or make the blade disappear. While the blade exists, this magic longsword has the finesse property. If you are proficient with shortswords or longswords, you are proficient with the Dusk blade.

You gain a +2 bonus to attack and damage rolls made with this weapon, which deals necrotic damage instead of slashing damage. When you hit a creature other then an undead or construct with it, that target takes an extra 1d6 necrotic damage.

The sword's dark blade emits darkness in a 10-foot radius and turns light into dim light for an additional 10 feet. While the blade persists, you can use an action to expand or reduce its radius of darkness and dim light by 5 feet each, to a maximum of 20 feet each or a minimum of 5 feet each.

Proficiency with a Longsword allows you to add your proficiency bonus to the attack roll for any attack you make with it.

Notes: Bonus: Magic, Damage: Necrotic, Replace Damage Type: Necrotic, Weapon Property: Finesse, Damage, Combat, Versatile, Sap