Thanks.
Well, as soon as they use a spell that targets, they'll see it for what it is. Maybe there could be a cool animation when it's unveiled so people don't think the creature suddenly changed.
And if you made assassinate have a cool animation with it (like a dagger slashing the creature or something) when used, I doubt people would think their creatures health is gone for a random reason.
Problems could arise with newbies that don't know all the cards, but as soon as they see the card in the bazaar (and I know I at least checked out all the cards when I started), they probably won't have any questions.
Another option is to make an AI or two that uses it, so newbies would have experience with the AI using it so wouldn't be surprised when it happens in pvp.
Yes, it could be a problem. But I think the idea is cool enough to warrant the risk
.