Eldritch blade

You brandish the weapon used in the spell’s casting and make a melee attack with it against a creature within 5 feet of you. On a hit, the target suffers the weapon attack’s normal effects and becomes cursed by eldritch energy. If they attack you on their turn, they take 1d8 force damage. 

At Higher Levels. At 5th level, the melee attack deals an extra 1d8 force damage to the target on a hit, and the damage the target takes for hitting you increases to 2d8. Both damage rolls increase by 1d8 at 11th level (2d8 and 3d8) and again at 17th level (3d8 and 4d8).