Wistful Rex

{3},{t}: Wistful Rex fights target creature an opponent controls. Put a regret counter on Wistful Rex.
In the beginning of your end step sacrifice Wistful Rex unless you pay {4} for each
regret counter starting from the third one.
I knew I shouldn't have eaten the third one. It's
bad for my stomach. But again, this tantalizing, sumptuous smell...