Let's start at a happy ending. Toriel Dreemurr, the queen of the underground, who's lived for about half a millennia, sits on her throne of gold, reading back on diary entries and love letters, written long ago. She flicked through the pages of a worn down scrapbook of happy memories with those young souls who are no longer young. Pictures of her and her new daughter, Frisk. But Frisk was now twenty and visits at least once a week. Pictures of her and her knock-knock buddy, Sans. But Sans was now married and had a child. Pictures of Papyrus, on his first day of being captain of the Royal Guard. But now he was much more experienced and knew to be slightly less forgiving towards humans. Pictures of her and the first few humans to fall, in the Ruins. But Emery, five-year-old daughter of Undyne and Alphys, was the one who Papyrus took on day trips to the Ruins whenever he could.
Old, old photos of that one boy who was always by her side; who was always there for her; whenever she was sad, he helped her get through it; whenever she was happy, he was probably the cause of her happiness; when everyone else in her life had left, he was there. Asgore was there. Asgore was always there... But one day, he wasn't, and no one was there.
Toriel sighed, closing the flower-patterned book, and her white, fuzzy feet touched the dainty, delicate buttercups beneath her. She turned away from the throne and slowly stepped towards the dark hallway which lead towards the barrier. Just outside the dark arch to the surface lay the tiny, forgotten dust particles, left behind as the King had died.
She could've sworn she heard frantic footsteps behind her: the frantic footsteps of a human driven insane by being trapped here for the rest of his life. And the only thing he had to do to escape was to take the soul of the last living boss monster...