Ninja World: I Have a Life Simulator - Chapter 39 - : Eight Gates Acquired?

A cataclysmic threat looms over all realms in existence, unnoticed by any.The Enigma Race, the harbingers of ruin, corrupts everything in their path, harvesting countless souls for their enigmatic Lord, posing a dire threat to all living beings. No realm is safe.While confronting invading forces from a foreign multiverse, Elias is cast out of his space-time continuum and into the chaotic depths of the Temporal Vortex, a location that has spelled the doom of many, due to a catastrophic accident.To survive, the once supreme Quantum Seeker, now weakened by a cunning invader, musters all his remaining might and, against all odds, transcends the Vortex, losing his body in the process. His consciousness drifted into the miraculous Mystic Multiverse, located in a different space-time continuum. Now, determined to return and save the ones he left behind, Elias vows to reclaim his lost strength.But as time passed, Elias discovered that the new realm isn’t as simple as he initially thought…

Su Ran, a music prodigy, woke up transmigrated into a book.

At that point in time, the main story arc in the book had already been completed. As a villainess female supporting character, not only did she need to take over the original owner’s pitiful life in poverty, she also gained an instant son right at his rebellious period.

Ten or so more years from then, the son would turn into a twisted, evil villain. He would appear in one of the extra chapters to pick on the male and female lead’s children.

***

Su Ran: Son, you shouldn’t be too outstanding.
Su Han: Why not?
Su Ran: ’cause you will be killed.

Su Ran: Son, would you like to learn music from me and become a zen, little fairy boy who only needs to drink dew.
Su Han: No, I want to go into business so I can support you.

- Description from Novelupdates

After the final high school exams, Zheng Qing didn't go to the long-desired Imperial University. Instead, he packed his bags, hugged his fox, followed an enigmatic notice, and ventured into an unknown university.Here, there was no advanced math, English, or political philosophy; they were replaced with divination, astronomy, and the philosophy of magic.Extracurriculars here involved demon hunting,And the hobbies were curious and diverse.He would sail the sea of learning and climb the mountain of books.This bewildered young man started his tumultuous journey as a wizard.(Group created: Six Two Three Eight Five Eight Nine Three One, you're welcome to join.)

The strongest Demon Lord dies, and reincarnates as a human. Not only that, he because of various incidents turns into the hero’s best friend.

Follow his journey as he tries to help the hero escape his fate, and on the sideline to conquer the continent while he’s at it.