Perhaps the dog is magical and transports the reader to a world where Underfell Sans exists. Another possibility is that the dog is being chased by something, and the reader and the dog run into Underfell Sans while trying to escape.
Maybe the dog leads the reader to a mysterious place where they stumble upon Underfell Sans.
It could be that the dog gets lost and the reader follows it into a strange realm where they encounter Underfell Sans. Or perhaps the dog is somehow connected to Underfell Sans and brings the reader to him.
Maybe the dog led the reader to a place where they crossed paths with Underfell Sans by chance.
First, understand the character of Underfell Sans. He's different from the normal Sans, more edgy and with a darker personality. Then, create an interesting scenario for the reader to interact with him. For example, a dangerous mission in the Underfell world.
One great 'underfell sans x reader fanfiction' is 'A Dark Encounter'. It has a really interesting plot where the reader is thrown into the Underfell universe and has to navigate their relationship with Sans. The story is full of tension and character development.
Frisk would be a likely candidate. Since Frisk is the main character in Undertale and has a very adaptable personality. They could approach Underfell Sans with an open mind and try to understand him.
Well, in a 'underfell sans meets underswap fanfiction', it might involve a clash of cultures, so to speak. Underfell Sans is from a darker, more dangerous world where survival is tough. When he meets the Underswap characters, who are often more laid - back and kind - hearted, it could lead to some really cool storylines. For example, he could learn about friendship in a new way from the Underswap Sans, or get into trouble while trying to adapt to their more peaceful world.
In such a fanfiction, there could be a lot of conflict. Underfell Sans is from a more hostile AU than Undertale. He might initially be distrustful when meeting the characters from Undertale. For example, he could misunderstand their kindness as weakness or a trick.
The character development of Sans in this type of fanfiction is usually centered around his journey from a victim of abuse to a survivor. Initially, the abuse has left him with deep - seated insecurities and a negative view of the world. However, as he is exposed to positive influences, such as a caring friend or a new opportunity, his character begins to shift. He might start to develop new skills or interests that were suppressed due to the abuse. His relationships also change, from being guarded to more open and trusting. This development is often a slow and arduous process, but it is what makes the character so interesting in these fanfictions.
Typically, in these fanfictions, Sans's character is shown with a great deal of internal struggle. He is constantly fighting against the effects of the abuse. He may be depicted as having mood swings, sometimes lashing out because of the pain he has endured. His relationships with other characters also become more strained due to the abuse, and he may have difficulty opening up or showing vulnerability.
Start by making a list of their distinct characteristics. Underfell Sans has a more angular appearance and a darker color palette, which can be reflected in his personality as being more cynical. Sans, on the other hand, is more approachable in appearance and personality. When writing their interactions, use these traits. Let their body language speak volumes. Underfell Sans might cross his arms and glare, while Sans slouches and gives a lazy grin. As the story progresses, these characteristics can be tweaked based on the events they experience. If they face a great loss together, it could make Underfell Sans more vulnerable and Sans more serious.
There's 'Sans - Bound'. This fanfiction focuses on the challenges they face when they are forced to work together. It's not just about their relationship but also about the world - building in the undertale and underfell universes. The descriptions of the settings are vivid and add to the overall charm of the story.