The creators had an interesting idea, but unfortunately missed the mark with this film. Linking the Slender man Mythos with the Wendigo is a creative choice, and the idea that slender man infects others to become a slender man themselves, is a unique twist on the urban myth.
However, several things about this film make it fall far short of what it could be. The mockumentary style is a good choice, but most of the actors don't deliver anything that feels like people reacting to a real situation. Even in the last act, some are still reacting like people told "Okay some really scary stuff is happening, go!" than people actually caught in the middle of a supernatural invasion. The film does a good job of remembering what makes slenderman creepy at first (the less you see the creepier it is) but seem to have forgotten it by the end. The bad use of bad special effects (Between the dreadful photoshop quality tentacles, and the "creature" burning in the fire that looks like a paint program stick figure.) they loose sight of the lesson of web series like Marble Hornets. If you can't afford top of the line special effects, then don't try and do them.The bad quality makes the ending laughable, rather than frightening.
The reading of the titles on the screen too ends up corny rather than frightening. Better to have silence and let the audience do the reading. Far more unnerving. A few of the actors do make a good show of it, and until the last 20 minutes or so they do a good job of atmosphere. But the build up is far too slow, and the payoff is a big let down.
I'd advise this group of film makers to keep trying. There were some things here that polished the right way could have been a creepy little film. But the overall effort fails badly.