MTL - The Return of the Black Technology of Immortal World - Chapter 11 himself

To reward Meng Fan for his bravery before his heroic death, the cosmos rewarded him with a ‘system’, sending him back in time to three months ago, alive.

Amakawa Haruto died before he’d the chance to reunite with his childhood friend who disappeared five years ago. Rio, a young boy living in the slums seeks revenge for his mother who was murdered in cold blood in front of him when he was only five years of age.

There exists the earth many knows of and there exists an alternate world. Two people, each with their own backgrounds and values. For odd reasons, the memories and personality of Haruto, one who should have died resurrects in Rio’s body. As the two are confused over their memories and personality fusing together, Rio (Haruto) decides to live in this new world.

Along with Haruto’s memories, Rio awakens an unknown but special power. If he’s to use it, it looks as if he can live a better life, but before that, Rio encountered a kidnapping which results in involving the two princesses of the Bertam Kingdom.

After saving the princesses, Rio is given a scholarship to attend the Royal Academy, a school for the rich and powerful. His background being a poor orphan who once lived in the slums, the school full of nobles was rather a detestable place to stay at.

However, he did not know that he was not raising just a harmless pet, but a biological weapon secretly made by the government… a real monster.

As a former librarian, Rand felt that he had been living an ordinary life, the one that he had reclaimed unconsciously every single day.

Recently, he has been a little upset because the pet fish he accidently brought home to raised seems to have undergo some strange changes

Well, it turned into a merman.

- Description from Novelupdates

An underprivileged prodigy, a cruel, ruthless world, and an epic story of battle and conquest. A young man with an exceptional inheritance leaves the small, rural city from which he hails on a journey to find the girl of his dreams. He defeats countless prodigies, sweeps through all forces that dare to stand in his way, and progresses step by step toward the pinnacle of all the worlds!
“I’m not targeting any gods in particular. All I’m trying to say is that all gods are trash in my eyes!”