Darkend Strike

The next time you hit a creature with a weapon attack before this spell ends, shadowy tendrils grasps the victim you strike. The attack deals an extra 2d6 cold damage to the target, who must take a constitution save. On a failed save, the target can not take any reaction until the end of your next turn. 

At Higher Levels. When you cast this spell using a spell slot of 3rd level or higher, the extra damage increases by 1d6 for each slot level above 2nd.