The World After the Fall

A brief description of the manhwa The World After the Fall: [From the authors of Omniscient Reader’s Viewpoint:] Humans were suddenly summoned to become “Walkers”, and they needed to clear the tower to save the world.
Then, the “Stone of Regression” was discovered. Walkers could now “return” to the past. And slowly… everyone left. Humanity’s last hope, “Carpe Diem”, was formed by people who refused to abandon the world.
But once the last Walker reached floor 100, he no longer knew what to believe. This is the tale of the only man who didn’t regress, despite everyone else going back to the past.