Undertale has a extremely large fan content base, one concept that is popular is a thing called Alternate Universes, which are universes altered in some way from the original game. One popular AU is Underverse by Jael Peñeloza, which involves multiple popular AUs within the fandom, plus Jael's own AU, Xtale. It is a very popular series and it is animated with a full OST as well. It currently has 18 episodes for Underverse and Xtale has 11.