Undersided
Undersided (initials EUC or PAN) is a project intending to extend the world of Undertale by Toby Fox, doing so by adding new cultures, locations, history, and characters. It is not set on Earth. It can be seen as an alternate universe of Undertale. Undersided takes inspiration from the worldbuilding of several JRPGs, mostly Final Fantasy XIV. As a result of this, since nobody speaks English in-universe, many characters are given new names corresponding to what they are called in-universe. However, the story of Frisk in the Underground is as similar to Undertale as Undertale's English and Japanese versions are to each other.
Format
Undersided is mainly exposed through a number of entries hosted on the Undersided website, which describe the world of "Yona" from an in-universe perspective. Alongside this, there are also a number of non-diegetic resources, such as images, audio, and other media, as well as the wiki itself.