First report of Slenderman figure; also called the "Nephilim" in the Book of Genesis (6:4):
(Translations according to New International Version. Note that translations frequently differ. In the King James Version of the Bible, "Nephilim" is translated as "giants" in the following examples.)
The term "Nephilim" occurs just twice in the Hebrew Bible, both in the Torah.
The second is Numbers 13:32-33, where the Hebrew spies report that they have seen fearsome giants in Canaan:
32. And they spread among the Israelites a bad report about the land they had explored. They said, "The land we explored devours those living in it. All the people we saw there are of great size.
33. We saw the Nephilim there (the descendants of Anak come from the Nephilim). We seemed like grasshoppers in our own eyes, and we looked the same to them."
Perhaps you are skeptical. Well, let me explain.
Slenderman is really a son of God. You can consider him like an agent. In the image you see on the actual Slenderman article page, Slenderman bears strong resemblence to that. Aliens, in various contexts...
Have been said to, "take the form" of humans. In this manner, the real image of an alien is in fact, totally unknown. Common perceptions of alien life look like:
Many may judge that the alien seen here do not strongly reflect Slenderman. Some many even consider this an "insane" comparison. The simple truth is spitting you in the face. If these lifeforms can take a human shape, then they really are the proposed Nephilim. Their true shapes can reflect humans... very much like the Slenderman image you can view today.
Something only a Son of God could possibly do - with superficial magic, or highly advanced technology.
Slenderman's real "name" as one of the Sons of God is Death. He will follow you when you sleep. He will hear you when you are awake. He will be in your dreams. His existence and being read and discussed only helps shape his form.
I hope you don't give into fear like myself.