I cut what I have to say in parts so it isn't something people will just tl;dr because its not a huge wall of words.
Its just the slenderman addon with some simple video effects behind it. And the default mansion. Eff was almost completely right.
I don't find this scary because I've done so much work with those mods myself that I know its ins and outs.
However, to make it scarier, replace the sound effects possibly. Also, add some more creepier ambience in. The ambience I heard in the video was good but it wasn't too bad- you want something that sounds more hell-ish.
At the end when the thing got you, try to make it more of a jump, as in making it louder, instead of zooming in and adding a few TV effects.
Another suggestion is to make the video darker and to use some friends as actors to make it less predictable (because you used the mod.)
Nobody usually reads what I have to say but there you go; my opinion and how I would suggest to fix it.
