The Au starts as a changed HorrorTale!Au although it's less evil. Monsters would eat other to live, Having wiped out the Vegitoids. People ate them their magic turning to energy. Having to eat as much as you can before they dust, some would use healing magic just so they could eat all of them. In my au once monsters get far enough into starvation their body starts mutating trying to give it the need traits to protect itself but like most mutations irl actually harm them. Undyne overthrows toriel and becomes queen genuinely trying to help the people. Since sans doesn't lie about her saying to eat humans in this the only monsters that try to eat Aliza are ones doing fully of their own choice. Like in cannon alphys tells undyne that sans's eye can be used for the core and undyne at first refuses. But the stress of her people dying made her go through with it, one eye to help hundreds. Sans's scars them both with blasters but alphys is not lobotomized. Since frisk is still alive on the surface they are the only one who can reset. Flowey shows where he hid the souls when they left him but couldn't leave the barrier as frisk hadn't helped them and just left. Both Sans and Flowey encourage Aliza to take them. But she still needs a monster soul so sans lets her take his cracking a joke about how body has been used to help monster kind twice now. They shatter the barrier and let the souls leave to rest having accepted that sans was going to be stuck together. But the soul of kindness uses the last bit of their power to pull his soul free with them. Leaving Aliza and sans separated and monster kind free at a cost to his health and magic.