This wiki contains spoilers for Undertale and Akintale, including info not yet covered in the series.
Sans, Papyrus, and Alphys comb the archives for an ancient scroll. Once found, they search the scroll for the rule of ascension to find out who the next king will be. After a long search, they discover that, by process of elimination, Papyrus will be the next king.
The Wingding Diaries: Entry Number Two
Here we get to see the followers for the first time and get some fun with a mischievous kid Sans. We also get why Gaster never fixed the Reality Tear Device and the inspiration Gaster needed to create the Dimensional Boxes by copying Ace's pocket.