I just received an email from Fuei Shokai (腐影商会) with a bit more information about their Slender Man release:

The first figure from Fuei Shokai, released at Super Festival 57, is their interpretation of Slender Man.  Slender Man is present in American urban legends, but their interpretation draws from an older version.

This iteration of Slender Man is based on an 18th century German poem Erlkönig; literally translated as “alder king.”  The main character of the poem finds himself lost in the woods.  His death marks his transformation into Slender Man.

This is where the paths of the urban legend of Slender Man and the poem cross.  Slender Man wanders the dark in search of children.  He is invisible to adults, but children are able to see him.  He may also appear in children’s dreams.  Slender Man is responsible for child abductions that are typically written off as runaways.

Next week, Fuei Shokai plans to release a new deluxe version.  For now, the first release can be purchased in the US exclusively from Lulubell Toys.