The First and The Last (A Creepypasta X Read...
This is a story about Slenderman's first proxy. Now, of course, this story must start at the beginning, when he first meets you. Set in the 1800s, you meet Slenderman after running away from some men and into the nearby forest.
After about 200 years, when Masky, Hoodie, and Toby are his proxies, he assigns them a difficult task. A task to find this "runaway proxy" and bring her back. But when they do, they realize they've got a very, very hard decision to make, and...well, I wouldn't want to ruin the plot.
Also, I want to thank @Taylor_loveDun for helping me out with this book!