Alright, I think it's time for me to start covering up my cards, isn't it? As we all work together for the same goal, for none of us knows his remaining time, I decided to trust you.
Before I start to explain my theory, I would like to intodruce myself once again. For reasons of internet security, I cannot tell you my surname, but for now, you can call me Leon. I am an almost 17 year old student from Germany, Lüneburger Heide, Lower Saxony. My interest in the occult started at the age of 5, my first contact with the Slender Man was when I was 14. First starting by watching Let's Plays of the game, later I took up my old studies and began to inform myself about Slender Man. To the very beginning, it was harmless, I read some things up, wrote some Creepypastas.
But then, I came across the Operator Mark, which is in my opinion the point in ones life, when he enters the dangerous material already, and soon realized, that my physical condition, which was always very weak, grew worse, I was sick, tired and felt helpless. Shortly after, I started to worship Slender Man as a god, wishing to be his Proxy. I broke free after a while, but that was when it grew worse: I developed something like a multiple personality, a boy named Christoph/ Christopher Deleu. I never realized it was me, to me he was a abducted victim, who left lots of cryptic notes and notebooks.
I started to post his notes on this Wiki, even a long time, after the sickness faded and Deleu was gone. At one point of time, I was banned by Foba for six months. Since I had a lot of time, I used it to recover from my sickness completely. After that, I took my first and second name, Leon and Christoph and took the second for first. Then, I translated Leon into german, where it means Löwe, or, in old german, Leu. So, I recreated the name Christoph de Leu/ Christoph Deleu. After I realized, that it was all me, I checked up all the Deleu notes again and reconceptet everything I knew. After a while, my whole memory returned, those as Leon and those of Deleu.
Now I returned to state my theory about Slender Man:
There are at least two different dimensions. One is ours. The other one is of a different kind, it is different from everything we know. The dominant species of this world is the creature I reffered to as the Faceless Ones, even when I was Deleu. This species is surperior to us, at least on all intellectual bases. One of their abillitys is to create incarnations of theirselfes, beings made out of their mental power, which they can even send into our world. They're ultimate goal, however, is to create a gate, which they can use to enter our realm themselves. Therefore, the Faceless Ones experimented for milleniums to find the right ritual. Slender Man, also called Gorr'Rylaehotep, Nyarlathotep, Ritter, Grossmann, Tintman, Fear Dubh and probably many others, seems to be the most successfull and most likely also a very powerfull member of his species. If he weren't the most powerfull one to beginn with, his succes might by caused by the fact that he is the first "god" to understand the human network of society, or, to be more specific, the internet. Since his behaviour changed in the last few years, I would guess his experiments are over and he is now preparing the ritual to open his gate.
Feel free to ask questions or at least to post your opinion to this theory.
As for the real form of the Faceless Ones, there are four theories I would consider at the moment. I will, however, start with the most unlikely:
1. Nyarlathotep: As I think that Lovecraft might had contact to their world, his Old Ones might be based on creatures of this realm. With the attricbute of a missing face, Nyarlathotep would be the most likely to create Slender Man. He is also mentioned to create incarnations to act in our world, just as I stated before. Furthermore, Slender Mans name "Gorr'Eylaehotep" has much similarity to Nyarlathotep. Its possible that the Faceless Ones might look like him.
2. As in our world: I, for myself, think it's rahter unlikely that a creature from an other world wears a suit or has this much in common with our world. But then, Number 1 is based on one horror writer, so I decided to consider this as more likely: Slender Man might just look as his incarnation in this world. Transferring his own picture could possibly be easier than think of a form on their own and to create it by thoughts.
3. A tree: It might sound funny, but there is also a chance that Slender Man is, well, a plant. Since he lives in an other universe, sentient botanic live could definitly exist. Many old cultures worshipped certain kinds of trees as sentient gods. So why not? It would also explain Slender Mans affection to forests, his connections to trees, the way his limbs look sometimes like trenches... and it would explain his mental abillity. Sounds strange? Well, think about it: If you were a sentient tree, a sentient being WITHOUT the abillity to move, wouldn't it be naturally to develope abillitys to act with more reach? The abillity to project his incanartions could be the trees way to act without moving. It would also explain, how he can be there for ages, for trees can be thousands of years old. I, for my part, would like to think of the oak of blood tears from the fear mythos, but I don't think that is too likely. It would possible be more like a Ceiga tree, but it is most likely a species of trees we cannot compare to anything in our universe.
4. No form: Yes, that is actually the one I believe in: The Faceless Ones are not only faceless but entirely bodyless! Who can prove that they are not just, well, minds, conciousness beings without body? The idea developed when an other user asked on this Wiki if all the known faceless beings could be aspects of one being. I thought: But why would one being act in different ways? They'd have to be seperate beings but somehow still connected... Well, without the limits of a body they conciousnesses (or whatever) would melt into another, seperate again, it would be like a giant cloud of minds. This could be another explanation for their great mental abillitys.
Your thoughts? I wait for them.^^
There is also a possibility, that the other dimension itself is Slender Man. That might sound weird, but imagine that every living being in this world might be connected. Slender Man could be the collective mind of all the living being. I don't think so, but I'm professional, so personal opinions won't stop me from sharing my thoughts with you.
About connections with other Slender Man- storys, there are two things which cought my attention. The first one is the Path of Black Leaves. If our realms are connected, there is most likely something like a bridge between our worlds. The Path could be this bridge. Slender Man uses it as an operative base, but he can't enter it physically, so he sends his Incarnation, just like in our world.Also, the connection to trees would support my third theory from Addition One, where I already mentioned parts of it. I will come back to this later.
The other one is the well known, yet not defined, Ark. I think the Ark is something like a key, used to enter our world. There are, however, two ways it can work:
1. The Ark is the key. It opens the gate and makes it possible to enter the bridge and the world beyond.
2. The Ark can be compared to Noah's Ark and is used to travel savely. I get the strange feeling that Slender Mans intentions are not the intentions of his whole world, as I said, the theory in the beginning of Addition 2 is not very likely, and that creatures from this world should not go to our realm. In any case, I think there might be something like a guardian of the bridge which will kill you if you enter it without the Ark.
About the Tree and the Fear Mythos: I will assume, that my theory Nr. 1 from the original post is correct. Also, the tree is on the path, not in the world beyond. Therefore, here is another theory: Slender Man uses saplings to spread his power. If there really is a guardian, which can hold Slender Man at bay, he would have already started to make sure he is stronger, when he gets to fight it. Therefore, he plants his trees on the path, making them basically batteries. More about this at a later point.
About the world of Slender Man
Its a bit small, but it's a little diagramm. As you may see, the other world is reffered to as "Sein Land" meaning "His land".