A mysterious pony delivers a briefcase to Twilight Sparkle. For Lavender Unicorn, the delivery becomes source of many problems, and because of threatening letter that came along with it, she must race with time to find a mysterious object called The Ark. But soon she will find out that she is in even bigger trouble.
——————
The humans will be mentioned just briefly, but still they are pretty important part of the storyline.
Crossover with Slenderman and it's universe. (And some other horror stories)
There are spoilers in the comments.