Nightmare found a young child a long time ago. When they fell into a coma it sent him over the edge. several hundred years later the child wakes up in a castle of darkness, surrounded by people, er... skeletons, that they don't know. Now they are being raised by murderers, cannibals, and world destroyers. Oh yeah! And...
You always thought of Sans and how cute he was all the time to his laugh to straight out personality. But things are starting to change, when Sans got in a fight with Papyrus, and your friend Muffet disappeared. Until you discover a dark truth about Sans and his love for you...
Sans has quite the collection of cracks on his bones, each recalling a memory, important or painful experience. If he wants to decide his fate, then he'll have to remember the scars and pain he's went through. May Become Dark, Suicidal, Depressive and Violent. But hey, it's Underfell, of course it's gonna end that way.