The item appears to be a longsword hilt. While grasping the hilt, you can use a bonus action to cause a blade of dark lightning 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 arcblade.
You gain a +1 bonus to attack and damage rolls made with this weapon, which deals lightning damage instead of slashing damage. When you hit a creature with it, you can cause lightning to arc from the target to a different creature of your choice that you can see within 5 feet of it. The second creature takes 1d6 lightning damage.
The arcblade is able to absorb ambient electrical energy to briefly enter a supercharged state. If you take lightning damage while this weapon’s blade is active, you can use your reaction to capture some of that energy and store it for up to 1 minute. The first time you hit with the arcblade before the end of the duration, the target takes additional lightning damage equal to half of the damage you took. This property can’t be used again until the next dawn.
Proficiency with a Longsword allows you to add your proficiency bonus to the attack roll for any attack you make with it.
This weapon has the following mastery property. To use this property, you must have a feature that lets you use it.
Sap. If you hit a creature with this weapon, that creature has Disadvantage on its next attack roll before the start of your next turn.
Notes: Bonus: Magic, Damage: Additional, The arcblade is able to absorb ambient electrical energy to briefly enter a supercharged state. If you take lightning damage while this weapon’s blade is active, you can use your reaction to capture some of that energy and store it for up to 1 minute. The first time you hit with the arcblade before the end of the duration, the target takes additional lightning damage equal to half of the damage you took. , Damage, Combat, Versatile, Sap