I Keep Reincarnating, Why Is He Still Alive? - Chapter 51 – Difficult to understand

Hit by a truck as a young man, after winning God’s ultimate lottery prize, I got to create a HOLY sh*t-like cheat ability of my choosing, create a harem, make loads of money, sleep with a random babe, and do anything I liked; it’s that kind of story.

A broken man who ended his own life finds himself reincarnated into the game he used to play. However, instead of becoming the protagonist or a powerful antagonist, fate places him in the role of a minor villain—a mere stepping stone for the protagonist in the game's first arc.

In the tenth year after the new era, the world finally came to an end. Zhang Mu truefully waited for his death, but unexpectedly found that he had returned to ten years ago. With his memory about the future, he swore that he would change the calamitous history, saving the earth from being destroyed! Walking dead, zergs, evolvers, and various mutated animals… nothing can stop him! The first step is to retrieve his identity as an era merchant , then find all the treasures he can remember to enhance himself!

- Description from Novelupdates

This legend tells the tale of the protagonist, Kuroki summoned to protect the demon king from the hero.

Kuroki, unbeknownst to him, suddenly came to another world. He was summoned by the demon king, Modes. The territory which the demon king overrules was in the midst of being destroyed because of the hero. He had no choice but to bow before Kuroki.

“Please save us, Savior Dono!”

Kuroki nodded, deciding to save the demon king due to an unexpected turn of event.