First of all, a few masters started to vanish without any warning. It happened sporadically at first, and everyone thought they just got to a place away from the reach of the Message Paper coverage, or perhaps they stumbled upon sealing arrays or something.
But soon enough, more masters started to send distressing messages, speaking about weird stuff before losing contact with them.
Once William got the news, he instantly called off all the scout activities. He then shared the news with every one of the various forces, and they all agreed to wait before all the sides got cleared.
Then they'd all move together, closing the net, and exploring what was going on inside that area.
William waited for another day for the hills to get removed. The moment the last hill fell, and even before the dark master armies would be pushed back deep into the region, something grand happened.