Time, Arkham Batman thought. If they wanted to complete the remaining seals at the same time, then without any means of long-distance communication, the hardest problem to solve was how to act simultaneously.
Even if they set out from the same place, the distances to their target locations were different, so the time they took would be different too. If they acted the moment they arrived, Schiller would probably still have time to hunt down the others. Once someone went down in that time gap, the chances of completing them simultaneously would be low.
In other words, they needed to agree on a signal. After reaching the designated locations, they could only begin once they received the signal. That was the safest method. But the biggest problem was that they were too far apart. They had no way to communicate, and they had nothing like signal flares, so how were they supposed to give that signal?
But the instant he saw the lightning, Arkham Batman had an idea. He wasn't sure whether this was something Doctor Manhattan had designed on purpose, but while a thunderstorm night caused trouble for the Exorcist side, the lightning could also be used as a signal to act, because it was a signal the entire map could see.
As long as they agreed that after reaching their target locations, they would act together at the next flash of lightning, then there was a good chance they could complete the seals at the same time.
Arkham Batman calculated the intervals between the few flashes of lightning he had seen. They were basically no more than twenty minutes apart. Though the castle was huge, if they started from the center, twenty minutes was enough to reach any location, so there was no need to worry that the