You should begin with a clear idea of what makes your Undertale AU different. Develop interesting character backstories and interactions. And make sure the dialogue is engaging and fits the tone of the comic.
Well, they might see different versions of characters from Undertale. For example, the RWBY characters could be really surprised by the unique battle mechanics in Undertale's AU multiverse. Maybe Ruby would be especially interested in the different types of weapons or magic used in that world.
Asriel and Chara have a complex and tragic story in Undertale. Chara was a human who fell into the Underground. They became friends with Asriel, the prince of the monsters. They had grand plans to free the monsters by using Chara's soul. However, things went horribly wrong. Chara's plan involved sacrificing themselves, but when Asriel tried to carry out their final wish, the humans attacked him, thinking he had killed Chara. Asriel was too kind-hearted to fight back and died as a result.
One important event is when Chara first fell into the Underground and met Asriel. Another is their decision to try and free the monsters together. And of course, the tragic event when Asriel carried Chara's body to the surface and was attacked by humans.
Blake could also be a great character to focus on. Given her own complex backstory and her tendency to be more introspective, she might have deeper insights into the characters and situations in the Undertale AUs. She could form unique relationships with some of the more morally ambiguous characters in the Undertale multiverse.
Au999.9 was pure gold, and the gold content was no less than 999.9%. It was the highest value in the naming of jewelry. It was dark yellow in color and extremely pure. It shone with golden light and was bright. The markings were usually printed with words such as "Thousand Gold,""999 Gold," or "G999."
An AU fanfic is a type of fan fiction where the story takes place in an alternate universe with different settings or circumstances than the original work.
An au fanfic is a type of fan-written story that takes the characters or setting of an existing work and places them in an alternate universe or situation that differs from the original.