The promise.
This is a backstory between Sans and Alice.
(If you haven't read the first part yet, I prefer reading this first. https://timetwist.tumblr.com/post/168184325003/first-meeting-this-is-before-sans-fall-in-the )
Alice kept her promise, Sans accepted to let her know more about him and the monsters remains in the forest (not all monster were sealed y'know, that's strange) many days later, they became best buds.
Months go by. She goes to the forest more often. Someone were curious and followed her. When they saw her playing with him, they immediately informed the village chief to go and kill the monsters hiding. After that, all of villagers attacked and killed the monsters with cold blood. while both of them ran as fast as they can to find a safe place, Sans were injured after saving her from getting slashed by a spear in the chest before getting away, Alice tries to think what she's gonna do, the only items she have in her pockets are a toy knife that used for playing and cutting leaves and bandages, but the wounds are deep so it cannot be help if only bandages but they have to move until they reach to Mt. Ebott. One villager didn't give up and followed them all the way there. They threw a spear that was supposed to hit Sans but unfortunately, Alice takes it. That moment, he was filled with rage and sadness. His only friend. dying in front of him. They made a promise at each other. The war between humans and monsters will end someday and she'll always stay by his side, he did keep that.
Before she dies, Alice offered her soul to keep her promise then starts to fade away. Her memories starts flowing through him and starting to feel dizzy and fell underground. And the human that attacked them? He wouldn't be able to survive that fall anyways. So their work here is done.