My Dungeon Life: Rise of the Slave Harem - Chapter 866

The end of the world is coming, the world has undergone great changes, and everything has changed. Plants, animals, and even insects, as long as there are living things, have mutated.
Become more powerful, and even evolved supernatural abilities, such as chickens and ducks that can fly to the sky, such as cats and dogs that can breathe fire.
But only humans, without any obvious changes, seem to have fallen from the top of the food chain to the bottom of the food chain overnight.
Any creature that can open its mouth to eat can treat humans as food.
But fortunately, this world has not completely killed humans. Gradually, humans discovered that they had the ability to cultivate compatibility with other creatures.
As long as it has a high degree of compatibility with some powerful mutant creatures, it can be used for its own use to achieve the purpose of protecting itself.
Fang He is a person who is very compatible with mutant animals, but it is also because of his ability that he has changed from the strongest to the stupidest.
Fang He, who was reborn, said, “Hmph, this time…”

- Description from novelbuddy

Our protagonist Caam is a man from Earth that was reincarnated into this world as a demon. In order to survive, he spared no effort in avoiding the sparks of battle and without noticing it, he became strong himself.

Our protagonist, once a homeless child navigating the city's harsh realities, clawed his way towards a better life. He stole coins to buy knowledge. However, the prestigious university scholarship he craved was a rigged system, awarded to a politician's son who needed the name, not the education.Descends into a life of crime.After being forced to taint his pride his death is ordered with his own making the final blow.god “congratulations you're given a chance at a new life...”him “ why?”god “cause you had a tough life so im being generous... and making you reincarnate in a mana world...”him “ bullshit...”On the brink of oblivion, a dubious offer arrives – a second chance from a strange god. Haunted by the shadows of past 'generosity', Rei struggles with suspicion. Accepting means becoming a pawn, rejecting means eternal damnation. With no good options, Rei plays along, unaware his role thrusts him into a cosmic conflict.

She was a medical expert, talented and beautiful. She was reborn as his little wife. Her in-laws did not like her, her husband did not love her, and she was as timid as a mouse. She was weak and easy to bully.