The Box, for certain. The door requires the full heart. Plus...I've just said it-in order to complete the ritual, we need the essences of the six Stalkers. We can't wait-and Mew's powers ought to help keep us from horrific and sudden Stalker death ourselves. I think they may need time between each of their ritual killings in order to make everything 'proper,' but the instant that time is over they strike again-no time to lose.