The Path of Black Leaves is the domain of The Slender Man and is commonly used by Slender Man's Proxies. It is also where The Bleeding Tree is located.
The first appearance of the Path of Black Leaves was in White Elephants, where Robert Sagel found himself on the Path. He described it as a "Dead quiet street. No one in sight. The trees on the sidewalk are filled with Black Leaves." Later on, the Path was described as a full on forest where strange creatures are said to roam and savagely attack those who do not bear the Slender Man's Mark.
Loops and Labyrinths might be extensions of the Path used to trap victims.