Black Flame Blade

A weapon you touch is shrouded in wisps of black flame. For the duration, when this weapon hits a target, it deals an additional 1d6 fire damage, and on the target's next turn, they take necrotic damage equal to your spellcasting modifier (minimum of 1). 

At Higher Levels. If you use a spell slot of 3rd level or higher, the additional damage dealt by this weapon is 1d8 + your spellcasting modifier, and on the target's next turn it takes 1d4 + your spellcasting modifier necrotic damage.