Although billed as "Slender 2" on some articles and videos, Sanatorium is an unofficial sequel to Slender, developed by a different company, but using the same general idea and gameplay mechanics.
As the name would suggest, Sanatorium takes place in an abandoned long-term health care facility, likely one designed for mental patients. The player needs to collect 8 pieces of paper ("pages") whilst evading the Slender Man.
Unlike Slender, Sanatorium is only available for PC. Sanatorium also features more maps to play on such as "Hospice".