Depths of the Abyss

Kotaro, known as the world weakest challenger, he was used as the bag boy, filled with anger and desesperation he could only stick to the strongest group that made him reach the last floor of the tower, ''Last floor reached, world end protocol starting.'' was said by a mysterious calm voice, everyone freaked out while Kotaro just accepted his destiny, what he didn't know it was that the necklace his dad gave him, would be his life saver, he returned 20 years back in time right before he became a challenger of the tower, filled with rage and desire of vengeance he promised himself he would kill everyone that made his life impossible.