Battle Royale.

Aeternus watched their change in emotions with glee. He is enjoying himself tremendously. The worst thing you can give a hopeless person is hope. They will do anything and give up everything for that hope. 

He chuckled to himself. "This is good. This is very good."

If he had said with certainty that he would free the last person, they wouldn't have believed him. They will understand his words to be an attempt to rile them up. They won't believe him because they think he is toying with them. 

But he offered uncertainty and with it came hope. Now they don't know if he is toying with them or not. They don't know if the hope is real or false but it is hope. It is the speck of light in an otherwise dark dungeon. They will grasp hold of this opportunity so that they won't lose it. After all, there is nothing to lose but their lives itself. They are going to die anyway. But by fighting well, they stand a chance of surviving.