Primordial Chaos Dragon Tower: Harem System - Chapter 815: The Humming Void

A spin-off of Lillia and Treize, Meg and Seron is a high school adventure/mystery/romance/comedy of sorts taking place in the rough equivalent of the 1950s. The story follows Seron, a stoic and handsome bookworm who is in love with his classmate–a sweet, oblivious transfer student named Meg.

Meg and Seron takes place at around the same time as Lillia and Treize, but knowledge of the main series is not necessary to read Meg and Seron.

Travel across different worlds and become a fallen aristocratic lord. With the help of the proficiency panel, you can start your liver experience with the ancestral knight breathing method, follow the wizard’s footsteps step by step, and embark on a bizarre and mysterious journey.

- Description from novelbuddy

Seven billion humans living on the Earth time-traveled together after they had woken up one morning. They all heard a synthesized female voice.

A lecherous army special task force soldier from 21st-century Hong Kong is selected to travel back in time, choosing to go to the Chinese Warring States Period. Unfortunately, the time machine runs into an error during the experiment. With no foreseeable way to return to the present, he must use his combat skills and knowledge of history to thrive in his new life in the year 251 BC.