Alright, so Undertale, yeah that game with good music. How strict is it with copyright?
I’ve seen games that uses sans as a character and megalovania as a song like A Universal Time (a popular jojo game) but it isn’t getting copyrighted. So is it safe to use Undertale stuff like sans or not? I’m sort of considering putting sans in my game.
It’s never a good idea to use assets from other games as there’s a decent chance you’ll get a DMCA request.
You could do your own research or touch down with the developers on this topic. 
Other games don’t get immediately taken down just for the inclusion of copyrighted material, but you should not make the assumption that it’s allowed just because that’s the case. Roblox doesn’t police content; they need a reason to issue a takedown. If that reason isn’t provided, they will (like anyone should) assume positively that you’ve sorted out permission for the use of content that you do not own.
Just in general, the same question gets asked every day with a different point of interest, so the answer is the same for anything. If it’s not explicitly public domain or open source, then get permission. If you don’t have permission, the highly likely answer is that you can’t use it.
“Toby fox is chill, but you should be careful with undertale AUs”
-quote from ImTheBuildGuy
in conclusion, Its fine if you use undertale characters, but you should be more careful when using undertale AUs (such as Fresh! Sans, Last breath Sans, Underswap Papyrus, Disbelief papyrus for those of you who don’t know)
but most AUs cant copyright, since toby owns the characters.
You didn’t have to ping us for this. The person you should be offering your information to is the OP, not us, we’re giving perspectives on the matter.
My post is intended more to be a general standpoint regarding copyrighted material rather than something specific to Undertale since the DevForum tends to get a lot of very specific questions about the copyright of existing works. The post was made six months ago and anything can change in that time frame, including how a copyright holder allows use of their work.
Ultimately ImTheBuildGuy is not a spokesperson for the creator of Undertale and their word shouldn’t be taken at face value without substantiation (as in, a credibly sourced statement from the IP holder themselves). Information should be taken from a reliable, substantiated source.
The “AUs” you reference are derivative works of Undertale and would reasonably fall under the same permissibility as a Roblox game unless said Roblox game isn’t creating their own “AU” and rather directly creating an Undertale experience on Roblox or using parts of the game.
Obligatory IANAL.
sorry for this… didn’t know you were against pinging
ive checked official posts, only selling undertale merch is an abuse, and can be taken down, but roblox, you can’t sell merch, so that’s not the case
this one, you should check prob for each one