Only a 10% death chance would make this card too strong; you'd pretty much spam it every turn without too much worry. I think it should be cumulative, so that the first time you buff a creature, it has a 10% chance to die. The second time, it's 20%, then 30%, and so on. This would make a hard cap of 9 buffs per creature, and in practical terms you'd be awfully lucky to get more than a few on one individual creature.
Just to clarify, I mean like say you use this on a cockatrice twice, it's 10% death chance the first time, then 20%, and then if you were to on another turn use it on, say, a dragon, it would be 10% for that, because it's not the same creature. If you were to try it a 3rd time on the cockatrice after that, it would be 30%.