A deep, muffled rumble echoed from the direction of the crevices they'd just passed. The ground trembled slightly, sending fragments of stone down.

"What is going on?" the guard captain cried out, his voice tinged with alarm as he stared toward the entrance.

The entrance, which had allowed only one person to pass at a time, was now completely blocked. An enormous boulder had appeared there at some point, and no one had noticed it. It was as if an invisible giant hand had sealed off their escap