i used to love undertale with all of my heart, and i didn't really have a problem with it up until i was close to leaving the fandom. i will list off very triggering topics in the following part(s), and those topics are:
p3d0ph1l14
1nc3st
(more to come)
so if you are triggered by these topics, i suggest you don't read the undertale chapter(s), because those two will be a re-occuring theme.