The grim adage that anyone who enters the dungeon ultimately dies in it, proven by the returning crawlers.
Carl invoked the saying while counting the cost of the war. Over 50,000 Former Crawlers had come back to help, people who had already survived the dungeon and left it — and they came back to die. Only about 28,000 were left.
'Once you enter the dungeon, you will die in the dungeon.' It framed his refusal, moments later, to become the monster his audience wanted, and Milk's bleak aside that none of the departing warriors were really going home either.
The dungeon's fatalistic maxim, borne out by the volunteers who left and returned to die.