You hurl a ball of raw magic.  Make a ranged attack against the target.  On a hit, the target takes (1d8) force damage and must make a Constitution Saving Throw.  On a failed save, the target must subtract 1d4 from the next attack roll it makes before the end of your next turn.  

At Higher Levels.  This spell's damage increases by 1d8 when you reach the 5th level (2d8), 11th level (3d8), and 17th level (4d8).  When you reach the 11th level, the target must subtract 1d6 from the next attack it makes before the end of your next turn, if it fails a Constitution save.