When an anonymous man entered the pony world, ponies started going missing. Can Frisk uncover the mystery?

One day, Frisk got the news of a pony going missing. Since then, more and more similar cases have popped up. In fact, someone entered the pony world and started a genocide spree.

Being the pacifist she is, Frisk uncovers the mystery with her friends before the murderer can kill them. However, as more evidence was revealed, they all connected to a specific person. Someone who used to be Frisk's best friend. Will she teach him a lesson, or will she show mercy?