Anti-Coat of Arms.

I hate tribal decks sometimes.

I like the idea, but maybe too strong as colorless. This is already strong since it punishes non-tribal if the deck gets multiple copies of the same creature out. Standard gets tribal sometimes, and this is probably an autowin if it resolves.
My resolution would be a rider that doesn't let the power and toughness (or at least just toughness) go below 1/1. That being said, I do tend to fear things as overly powerful and can overdo drawbacks.