Brothers Run Rampant in the Apocalypse

Three brothers traveled to the end of the world because of an earthquake. When they first arrived at the end of the world, they learned that they had traveled through a book, and they awakened their own powers after struggling. The three brothers' abilities were valued by the protagonist of the book, the young marshal, and he recruited them, promising that he would protect their safety as long as he lived. The three brothers settled down in the end of the world, but the black hands behind the scenes have been eyeing their abilities. They continued to grow and upgrade among various forces to fight monsters, and finally defeated the big boss.