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Humans were warned.

The apocalypse was coming and anyone who survived will be able to move into a different world. Anyone who didn't survive… well, they would die. There was no way to stop the apocalypse, no chance of salvation for this world. Summoning angels wasn't good, either. The only path ahead was surviving.

After Mark's video went viral, all Humans had been given a new hope.

Its contents were—as long as you sacrifice an animal in your stead, I would help you survive this war. It was a promise given to intimidate the A-rank Hunters. 

By saying that, at least some humans will be tempted and will surely come to the mega-dungeon to throw out some sacrifices into it. To expand the mega-dungeon, native creatures had to be killed inside it. 

And what better way than to make humans do it for him?

Now that he had the cooperation of the A-rankers, Mark revealed that truth to them. He required sacrifices to grow the mega-dungeon.