This Wiki will touch on many of the most common portrayals and media in the Underfell fandom. As any Wiki based on fandom, it is constantly unfinished, and we contributors welcome new additions to most common portrayals. This is not a survey of every headcanon, as there is no concrete canon aside from the design aesthetic, but simply a brief of the most popular ones.
Underfell is one of the many alternate universes (AUs), or retellings, of Undertale, a role-playing video game made by indie developer Toby Fox. Underfell does not have a canon story line, but rather serves as a design aesthetic. Most commonly, it relies on a role reversal of the "kill or be killed" line spoken by Flowey in the original game. The monsters are depicted as overtly murderous, violent, or simply downright edgy. Following this reversal, Flowey becomes a helpful guide.