I have an issue with enchantments that will probably kill their targets as soon as they are attached. Why not just make them straight-up kill spells? I understand the occasional need to reduce a creature's power, but I think cards like Meishin, the Mind Cage are better since it's going to stay around longer. It may sound weird to complain about this type of thing since you're basically getting the same value as a kill spell (a card for a card), but these types of cards tend to have one of three fatal flaws: they often have really weak definitions of "X", they don't really do much, or they are overcosted for what they do. Kagemaro's Clutch makes "X" equal the number of cards in your hand, Ghoulflesh is kind of pointless, and Screams from Within costs triple what it should. I get the idea of weakening creatures, but you're just masking the problem instead of dealing with it permanently. You can put a pot under a dripping pipe or you can buy some plumber's tape.
Anyway, on to this card. It falls into the "Flaw A" category: "X" is very weak. It can only really be powerful in the late-game, but if you've gotten to the late-gate and your opponent has enough creatures in their graveyard to make this worthwhile, you've either: won the game or you've exhausted a few, better kill spells in order to get those creatures in that yard. Despite all this complaining, I'm not about to call this a terrible card; I'm just going to say that there are better cards. This will kill most small, utility creatures fairly early and it could kill a larger creature before it kills you while giving you the option to weaken it beforehand. My main issue is this: why try and mask the problem of a big creature by weakening it with this instead of just killing it with something like Murder?
Pros: Can kill sometimes, weakens at minimum
Cons: Requires creatures in opponent's graveyard