Raw

Demon King—"Do you know why I have been deemed the Demon King? It's simple, the entirety of Purgatory is my domain. Within my domain, I am unkillable. No one in ten thousand years has ever exited Purgatory, but unlike the others, you do not get the benefit of joining my horde."

Those words struck the two like a lightning bolt. Now they understood the reason for the black treads. Each attached thread produced sustenance for the demon king, and all living organisms in Purgatory had one of these threads with their numbers in the thousands. 

That meant that no matter how grievously injured the Demon King was, he could respawn over and over thousands of times. It was hard enough when they had to do it once but thousands.

"This world was never designed for escape. If you are, the final hurdle. It's a slaughterhouse," May uttered as an eerie feeling overwhelmed her. 

From the start, they were doomed to fail.