Ever heard of the creepypasta Slenderman? This mod brings all your deepest fears about him to life.
All you need to do is keep an eye out for Slender Shrines, randomly spawned structures that feature a slender spawner.
Watch out. He can be killed with any sword on Easy difficulty, but once you up the difficulty to normal or higher, you need a diamond sword or slender sword to kill him.
If you manage to kill him, he may drop a child soul, which you can use to craft slender armor. The slender mask gives you a higher chance of avoiding nasty effects he causes, such as blindness or nausea. Also, the static effect made famous by the Slender game for PC will slow you down if you look at him.
Slenderman can break all light sources, glass, doors, and trap doors.
Happy slender hunting!