The theme of redemption can also be found. Sometimes, the more negative characters from underfell might find a chance at redemption through their interactions with the positive characters in underswap. It's like they get a new perspective on life and start to change for the better.
One common theme is the exploration of differences. Underfell sans and underswap papyrus are from different universes with distinct personalities. Fanfictions often focus on how they deal with these differences, like underfell sans's more aggressive nature compared to underswap papyrus's more positive and energetic self.
Another theme is friendship or partnership. They often come together to face challenges in the Underground. For example, in many fanfictions, they join forces to defeat a powerful monster or solve a mystery that threatens their world. This theme of unity despite their differences is really appealing.
The contrast between the two characters. Underswap Sans is more laid - back and friendly compared to Underfell Sans who is more rough and edgy. When they interact in fanfictions, it creates a lot of interesting dynamics, like how they might clash or gradually understand each other.
One popular underfell x underswap fanfiction is 'A Tale of Two Universes'. It beautifully weaves the dark and edgy elements of underfell with the more light - hearted and swapped personalities of underswap. The story focuses on how characters from both universes interact and face challenges together.
There's also 'Unexpected Encounters'. This fanfiction delves deep into how underfell sans and underswap papyrus might interact. It starts with a chance meeting and then unfolds into a story full of twists and turns. Their personalities are really brought to life in this one. It shows how they overcome differences and form a special connection.
You can start by looking on Archive of Our Own (AO3). It has a large collection of fanfictions for various fandoms, including the Undertale AU crossovers like Underswap Sans x Underfell Sans. Another place could be Fanfiction.net, though the quality may vary more there. Additionally, Tumblr sometimes has users sharing or recommending such fanfictions in the Undertale fandom tags.
There are some. You might want to check fanfiction.net as well. However, the quality can vary a lot. Some fanfictions focus more on the relationship development between Underswap Papyrus and Underfell Sans, while others might include more action or adventure elements related to their world.
There's a really cool fanfic called 'FellSwap Encounters'. It delves deep into the relationship between underfell sans and underswap sans. It starts with a chance meeting and then shows how they gradually understand and support each other. The author does a great job of showing their different characteristics and how they complement one another.