This item appears to be a longsword hilt. While grasping the hilt, you can use a bonus action to choose either light or dark. Choosing light causes a blade of pure radiance to spring into existence. Choosing dark will cause a blade of pure darkness to spring up instead. Click it again to 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 Yin Yang Blade.

The yin and yang are the balance between light and dark, good and evil, etc…. When the blade is dark the hilt changes it’s color to white. However, if you choose light the hilt changes it’s color to black. This directly correlates with the balance of the yin and yang.

the dark setting is pure darkness which means no light can pass through it which makes it pitch black. so if you are in a room with a lot of light the blade will still be pure black.

When attuned you gain a +2 bonus to attack and damage rolls made with this weapon, which deals radiant or necrotic damage instead of slashing damage. When you hit an undead with radiant, that target takes an extra 1d8 radiant damage.

The sword's luminous blade emits bright light in a 15-foot radius and dim light for an additional 15 feet. The light is pure white sunlight. While the blade persists, you can use an action to expand or reduce its radius of bright and dim light by 5 feet each, to a maximum of 30 feet each or a minimum of 10 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: Radiant, Replace Damage Type: Radiant, Damage: Necrotic, Replace Damage Type: Necrotic, Weapon Property: Finesse, Damage, Combat, Versatile, Sap