Sans, Sans, and Papayrus

We all know Sans, Frisk, Toriel, Papayrus, Asgore, etc., but there are others. Underfell, Swaptale, Mafiatale, ErrorSans, etc. This is a story about all the timelines colliding. However, such things don't happen without... consequences. Our story begins with Sans, trying to bring back a loved one, one who fell into the Core...