As the Minor Gay Love Rival in Het Novels - Chapter 4.7

Shion, the ‘Hero of Darkness’. He was said to be the strongest hero ever. He believed one thing, and that was his current party members.

“I’ll defeat the Demon Lord with this party!”

That is what he swore to do, but a tragedy occurs. He was painfully betrayed by the girls that he trusted.

He barely held onto his life, but the incident left a deep scar in his heart.

He gave up on living, and while he was wandering around in a forest, looking for a way to die, a single monster appeared in front of him.

“Will you kill me, please?”

The monster asked Shion, who was still strong, despite losing his powers.

What Shion, who wanted to die as well, did was―

Crossing to One Piece world, he became Roger’s younger brother.

Mystery is the nourishment that keeps me from being ordinary.

It has been 20 years since he came to a world similar to the modern European background. With his early wisdom, Fei Lin learned a skill and finally became a decent person.

Originally thought that he would marry a wife and have children and live this life in a dull way, but found that there is another world under the shadow of the world.

In that world, there are occultists who practice esoteric techniques and master extraordinary powers.

It is weird to spy on humans in the dark and feed on humans.

There are evil god believers who worship the evil god, please the evil god through bloody sacrifices, and exchange power from the evil god.

There is an evil god who wanders outside the world, peeps at the world, and only infiltrates the power of scales and claws, which is enough to cause people to fall.

With the mystery as nourishment, he is no longer ordinary after 20 years of ordinary.

- Description from novelbuddy

A back-to-basics(?) monster evolution novel.