Overshadows cloak completely. It's slightly more expensive, but is stronger, can't be countered and can barely be prevented.
Lowering the reduction would make it closer to silence, while still being different.
EDIT: Now that I read the notes I think I got it (I suggest work on the wording a bit). Not being able to use your own abilities or buffing your own creatures is not always a disadvantage, which would make this card much more situational (weak vs a mindless rush, stronger vs some types of stall). It also moves the balance towards spells being even more powerful, which I'm not sure is a good idea.