Curse of Flaying

When targeted with this spell an enemy's Armor Class is temporarily lowered.  

A target you can see must succeed on a Dexterity saving throw or their Armor Class is lowered by -5.  Until they succeed on a Dexterity saving throw the Curse is in effect.  This effect does not stack.  The target's AC can't be lower than zero.

You also have Advantage on melee attacks against the target.    

At higher levels.  The effect increases by -1 with every additional spell level.