Cat Scratch Fever

You deliver a magically-infused scratch that leaves more than just a mark. Make a melee spell attack against a creature you can reach. On a hit, the target takes 1d4 necrotic damage, and must succeed on a Constitution saving throw or suffer disadvantage on the next attack roll or concentration check it makes — whichever comes first — before the end of its next turn.

At Higher Levels. The damage increases when you reach higher levels: 5th level – 2d4, 11th level – 3d4, 17th level – 4d4.