Black-Viper Blade

You brandish the weapon used in the spell’s casting and make a melee attack with it against one creature within 5 feet of you. On a hit, the target suffers the weapon attack’s normal effects and a snake made from black venom coils around it until the start of your next turn. If the target performs an action that requires an attack roll before then, the target takes 1d6 poison damage and the next attack roll made during the current turn is made with disadvantage (unless it is immune to poisons), and the spell ends.

This spell’s damage increases when you reach certain levels. At 5th level, the melee attack deals an extra 1d8 poison damage to the target on a hit, and the damage the target takes for making an attack roll increases to 2d6. Both damage rolls increase by 1d8 & 1d6 respectively at 11th level (2d8 and 3d6) and again at 17th level (3d8 and 4d6).