I'd Give My Soul to You: Underfell Sans FanFiction

At one time, the Underground was a nice place. Despite some vengeful monsters, most were harmless, kind-hearted souls.

...

When a human fell down into the Underground, everything changed. The royal family took him in and raised him as their own along with their only son, Asriel. When a fatal accident occurs, and both children die, the king of monsters declares that any human that falls into the Underground shall be killed. While most monsters disagree, and ignore the law, there are a few who will do anything to please the king.

...

Years pass, and a human by the name of Katrina falls into the Underground. She is the last soul they need to break the barrier and declare war on the humans who put them in their prison. However, when an unlikely love story breaks out between Katrina and a certain skeleton, her soul is much, much harder to obtain. Will he be able to keep it safe, or will he watch his lover die, just like she watched her brother...?