According to the Observer from TribeTwelve, there was a major event involving the Slender Man and the Collective that took place during the course of World War II, was not documented by historians due to its top-secret nature. It is said that this event was one of the most important events in human history.
The Observer has stated that there were several witnesses and accounts of the event; however, all those who witnessed the event were exterminated. All but one account of the event was destroyed: Sebastian's Journal, which is now in the hands of Noah's grandfather. The Collective is currently trying to get their hands on the journal, but for some reason, they are unable to.