The text only shows the main plot points, because if I were to describe everything in detail, you wouldn't see the plot for a long time, and by then I'd have burned out on this universe.
If you are interested in any parts of the plot, it is better to write in the comments/, I will try to tell you in more detail.
Also, if anything isn't clear, it's best to leave a comment.
I've broken these key points into two parts. I'll post the second part later.
I'll also tell you everything about Neon! Sans a little later.
I'll also warn you. Sans isn't the main character in the story, so he doesn't appear in key scenes all the time.
English is not my native language, so there may be mistakes. Sorry.
The world of humans and monsters lived in harmony until magic began to rapidly disappear on the surface. This impacted the energy of the dungeon. After lengthy discussions, Asgore and Gaster came to the conclusion that to save the monsters, they needed to create a special system, a dome inside the mountain that would preserve magic. Thus began the era of CORE.
Asgore insisted on hiring the palace scientist, Alphys. Gaster, meanwhile, began assembling a team of gifted individuals to create the perfect invention. In a quiet bar, Grillby's, he bumped into Sans. A conversation with the skeleton, who saw the world as a set of crackable codes, astounded the scientist. Gaster took Sans on as his assistant. Thus, the quartet of Asgore, Gaster, Alphys, and Sans began developing and creating CORE.
When the System was operating in pilot mode, it was unstable, but monsters had already begun registering. The passage to the human world was sealed.
Years passed until Chara fell into the Dungeon. Her unregistered soul struck the fragile system, causing the first major failure. This released the first Ghosts, digital glitches. Because the system was unstable, the Ghosts themselves were too aggressive, pursuing Chara, whispering, frightening her in reflections, driving her mad. Chara, ignoring everyone, decided to destroy CORE to stop the ghosts from following her everywhere. She tried to reach the core alone, but after frequent encounters with ghosts, hallucinations, and incessant fear, her final battle with a Ghost scattered her soul, forever binding it to CORE. (This was not a physical destruction, but a catastrophic failure in her soul's data transmission.) A barely noticeable bug remained in the system, a few dead pixels above the CORE control panel.
Many years passed. Young Azriel, having snuck into the registration room, began to play with the system. He was struck by a digital wave—a surge of unsorted monster emotions. This caused a micro-glitch. This caused a micro-glitch, unnoticed by everyone but critical to the system. CORE rebooted and, having become confused, considered Azriel part of the Ruin Reserve, and his presence elsewhere a mistake. Thus, Azriel became an eternal prisoner of the Reserve.
After Chara, other children fell. For the CORE prototype, each new human soul was a virus. The system didn't erase them, but isolated them in digital prisons. But by scanning them, it became infected with their qualities. The Courageous soul made the energy system risky, while the Kindness soul craved connections. Unwittingly, the six children became the architects of CORE's "personality." Gaster considered this process natural and leading to perfection. Sans doubted it, but, blinded by his mentor's authority, he convinced himself he was right. Alphys was horrified and tried in vain to dissuade Gaster from such "use" of children's souls. Asgore, still devastated by the situation with his son, didn't want to know the details. He gradually handed over the reins to his determined right-hand woman, Undyne. She, too, actively participated in the completion of CORE.
To finally stabilize the system, an incredibly powerful soul capable of becoming its heart was required. Gaster himself became this soul. He voluntarily gave his consciousness and essence to his creation, becoming the living core of CORE. The program operated at full capacity, but with a fatal programming error: the Oblivion Protocol. Striving for perfection, the system began blocking all negative, sad, and painful memories from monsters. Undyne, seeing this as a path to universal peace, did not stop the process, but forbade Alphys from interfering, saying, "It's for everyone's best this way."
After learning about Gaster's actions and the scale of the Oblivion Protocol, Sans became completely disillusioned. He left his position as a junior engineer and became a simple "garbage man"—cleaning up minor bugs and secretly searching for a way to fix everything.
I will publish the second part later. I hope I won't make you wait long.
The post may be updated(?)
