This item appears to be a rapier. While grasping the it, you can use a bonus action to cause a blade of pure negative energy to spring into existence, or make the blade disappear. While the blade exists, this magic sword has the finesse property.
You gain a +2 bonus to attack and damage rolls made with this weapon, which deals necrotic damage instead of piercing damage. When you hit a celestial or fey with it, that target takes an extra 1d8 necrotic damage. This damage bypasses any resistances or immunities fey or celestials would have.
Proficiency with a Rapier 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.
Vex. If you hit a creature with this weapon and deal damage to the creature, you have Advantage on your next attack roll against that creature before the end of your next turn.
Notes: Bonus: Magic, Damage: Necrotic, Replace Damage Type: Necrotic, Weapon Property: Finesse, Damage, Combat, Finesse, Vex
Q: is this a normal rapier that can make an over powered blade or does it turn into the overpowered blade?
I am sorry for the confusion. It functions as the equal but opposite of the sunblade. What I meant to put in it was - "This item appears to be the handle of a rapier. While grasping the it, you can use a bonus action to cause a blade of pure negative energy to spring into existence, or make the blade disappear. While the blade exists, this magic sword has the finesse property."
I was writing this at ~2am and simply forgot to put the clarification in.