First Theory

Note, I have not done all of my research as of this moment, nor even red everyone's blogs, but I'm giving some of my theories nonetheless.

On November 1, Zeke Strahm posted the following that supposedly answered how Slendy came to be in the first place:
http://helpingthosehelpthemselves.blogspot.com/2011/11/mary-interview-part-2.html

This would seem like a major revelation and the answer to how this thing came to be... except for one glaring error: Reach (aka Raymond Shaughnessey)
http://whatyouareinthedark.blogspot.com/2010/11/forty-years.html

Reach claimed to be his slave for forty years. If that's the case, then how was Slender Man restrained by the cult Mary talked about in the first place?

My answer: He never was restrained. Odds are that, in order to sew confusion into the ranks of the Runners, it either planted false memories in Mary's mind, or was merely going along with the cult's activities for its own amusement.

Long story short: Mary's probably got nothing to do with Slender Man's rise in power. I will address another origin theory in a future post.