Black Slash

You imbue your weapon with the Shadow Dragon Nox's power. It turns into a dark, shadowy blade. This lasts for 6 seconds. If you hit a target with this blade, the targets takes 2d8+5 slashing damage. If you miss, the target can take an attack of opportunity on you. You can only use it once per long rest. And after you use it you are too weak to even move for the next 2 rounds, but you can cast defensive spells that are under the 3rd level. To see if your attack hits, add your attack bonus for that weapon to your d20 roll.