so today I had the wonderful opportunity of info dumping the entirety of the undertale multiverse to my friends with no knowledge of the base game (and they actually listening and were interested???!!) ...and this made me realize how truly insane of a fandom UTMV has.
Like... everything we base our discussions on...all the work we pour our hearts into...is fanfiction. Fanfiction (bad sanses and star sanses, the whole X-event) based off of fanfiction (AUs) based off of a random indie game made by Toby Fox and Temmie Chang (Undertale).
Nothing is canon, which makes everything canon...and all work of these characters interacting are purely speculation. There are hundreds, if not thousands of random people online contributing to this massive collection of media, building on existing concepts and branching off each other like a huge mutated tree.
It's almost like tiers to an iceberg in the Undertale fandom. On the surface there is Undertale, the base game made by Toby Fox and Temmie Chang, with some type of normalcy where there is a strict (albeit debated) canon. Go deeper, and there are AUs, fanworks making parallel universes. They hold their own 'canons' and are usually created by 1 or more people. Then you go off the deep end and fall into the Undertale Multiverse (UTMV), where the characters (usually Sanses) break out of there AUs and interact with each other, even breaking away from the base game entirely to fit around the concept of a 'multiverse' where there are creations and we are creators. Canonicity is completely abandoned, everything is in the hands of anyone willing to make art through song, animation, writing, etc. And it runs DEEP.