Please precise, whether you mean "in play" as in card or "in weapon slot" as in notes. "In play" for me would mean "anywhere in play, including your creatures, opponent creatures and opponent weapon slot".
I don't like the idea of simply buffing or debuffing a creature. I suggest changing Zenith to -2|+2 and Nadir to +2|-2. So you can't buff a creature forever, but if too strong you can decrease his damage to 0.