Death March kara Hajimaru Isekai Kyusoukyoku (WN) - Book 15 - 15-21. Brains

As a useless spoiled wild child, Bai YiHan finally brought about the end of his family, chased away the one he loved and caused his own death. After rebirth, he decides to cleanse himself and work hard at being a good child. But a lot of things don’t seem to be as he expected…

Bai Pa + Bai Ma: My son’s so obedient. Someone must be bullying him. This cannot be forgiven!

Bai Big Bro + Bai Big Sis: If my little brother’s sensibility comes from his suffering, then I’d rather he be a spoiled brat forever!

Mu JingYuan: If HanHan hits you, you must accept it. Yet you dodged…The wind’s changing. Someone should be bankrupt to make HanHan smile.

Bai YiHan: …I was so wild in my past life. Your involvement are undeniable!

This story is speculative, speculative, speculative. Important matters must be said thrice. Everything is about speculative time-traveling.

- Description from Novelupdates

Ian Percus, the second son of a countryside Viscountcy.

One day, he received a love letter from the future.

‘If we don’t protect the future, the world will perish.’

With an ominous warning scribbled by someone

“Great Mr. Chen Yu, having been successfully elected as the leader of the human race last night, do you have anything you’d like to say to the whole world?” Chen Yu got out of bed, bewildered and still undressed, “I don’t know, I just woke up...”

The youngest Wiseman of the continent, Chu Tian, has traveled 30,000 years back in time to the age of antiquity due to an accident.

This is a prosperous era. Rare herbs, minerals, treasure relics and monsters can be easily found throughout the land.

However, this is also an undeveloped era. Communications, traffic, martial arts and technology are in its early stage of development. Prosperous cities lack lighting while its citizens are uneducated.

Our decisive MC using his future knowledge creates Magic powered light bulbs, magic powered telephones, magic powered cannons, magic powered computers, artificial intelligence, psyche internet. 30,000 years worth of knowledge allows him to create miracles and shine brightly in this era. Allowing him to conquer the land and sit firmly on the throne of the 10 realms.

- Description from Novelupdates