Monsters had recently been brought to the surface from their prison in the Underground. Frisk, the young human that had helped the monsters reach the surface, grew over the next twenty years. She was now an adult of the age of thirty five and had a young child. She had named her young boy Xenon. He never had the chance to know his mother or father when a war between humans and monsters occurred once again.
The war lasted three years. During the first year, Frisk and Flowey were killed. During the second year, Asgore and Papyrus were killed. During the last year, Alphys was killed. The monsters were torn from the surface, being tossed back into the Underground. Their hope and kindness were ripped to shreds as many had been killed or suffered from serious injuries.
Toriel assumed the role of being queen of all monsters seeing that her son, Asriel, was still too young to properly become king. She soon began hating humans because of what they had done to her ex-husband, to her friends, to monsters in general. The only human she vowed to still love was Chara, whom Frisk had revived from being soulless.
None of Frisk's friends had known about Xenon. The only monster to know about Xenon was the father of the young boy. The father had said nothing about the child as he was now trapped underground once again, and with more aggressive and hate filled monsters that were still the same but different. The father of the child remembered Xenon, but never mentioned him or his wife, Frisk.
One hundred pain filled years passed, and the monsters healed from the shock of being back beneath Mount Ebott. Once everyone was back to their lives beneath the mountain, they realized how happy they were to be back in the Underground. When they realized their hidden happiness, their hope grew ever so slightly.
Toriel picked Sans as the new Royal Scientist, who was happy to take the job so as to take his mind off of his deceased brother. Sans had watched his brother, Frisk, Flowey, Asgore, and Alphys die. He had collected small bits of their dust, collecting Frisk's blood since she was human and didn't turn to dust. When he had done this, he took their remnants of dust, or blood, and placed it into a small box. Each box was labeled with their name, and Frisk's was also labeled with a picture of her bright, determined, red soul. Sans kept these bits of his friends close to him, hiding them in his house in Snowdin so no one could see his depression.
The first thing Sans worked on as the new Royal Scientist was finding a way to improve life in the Underground, though that wasn't the only thing he was working on. He had side projects that he worked on when he was caught at an impasse on the main project. One side project Sans worked on most was trying to find out how to bring his long lost brother, W.D. Gaster, back into existence. Another side project Sans worked on most was an attempt to resuscitate his friends. He knew that the goals of both these projects were near impossible, but he still tried.