I can ask the team a question (or rather Levi-Weaver and Rev / Revolvius), how do you feel about horror au (not only horrortale)
That is a broad question, but it would depend on the AU itself and how well it is written.
Good horror AUs draw out the horror elements and bring suspense to the setting. Utilizing the True Lab, trauma of monsterkind, the War itself, and the perspective of the fallen children alone could make a good horror story. From there, it needs effective execution, a relatable protagonist, and a degree of disempowerment for them to work really well.
Contradicting the set-building or characterization is one way of killing a horror story. Think Horror Sans leaving the Underground, including his brother, to die because his own eye got ripped out. He acted against his character and care for his family for no big reason, and even lobotomized Alphys for coming up with the plan. Going against established character for shock value is not an effective storytelling scare to begin with, especially with the horror genre.
The other more specific thing that can kill the horror is not building up to scares. Constant gore and frights do not frighten anyone, it's the build-up and release from it that does.
In summary, it's about the execution of a horror AU's premise that matters to Rev and I. If done well and ideally with Undertale concepts, it can certainly work wonders.
While Lev is saying some stuff about what we thought will make a good horror AU (Not saying we will even try to make one, god knows we have no experience on making horror stuff).
My personal thoughts on horror AUs are dependant on the character design, personality, and powers. There's also supporting force behind it like a good tone-setting, color palettes, and of course the story itself.
If you want to make a horror in general then don't make it cliche like most creepypastas from the 2012 era. Trust me they do scare kids (including me back then) but once you get older it all just went south.
But yeah, for me AUs like FreakTale and HorrorTale are pretty good. Idk much about the rest of the horror AUs because I have no interest in them so I have no comments on the rest.
But these are just our opinions so take it with a grain of salt.