This wiki contains spoilers for Undertale and Akintale, including info not yet covered in the series.
Ace meets Frisk in Waterfall and notices that the Golden Flowers only grow where the dead are. And seeing the flowers are in Waterfall as well, he deduces that this place is a grave as well.
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.