First off, let me just say that all of this is real. I have no videos or photos, and some of this information was told to me by friends.
My first world I deleted. I was in creative, and I dug underneath me to find a large underground lake. Don't blame me, I was only a newbie back then. When I looked up, the space above me was covered in dirt. But when I tried digging back up to free myself, it wouldn't let me. Rage quitting, I deleted the world and created another.
I didn't understand why I couldn't break the dirt above me. All I know is my first moments in the game were strange.
Since my parents were divorced, I had to go to my mom's house the next day. I downloaded Minecraft and logged in, and soon I was creating new worlds on there, too. I had only 3 worlds. I don't remember any of them.
A few months later, I heard of Herobrine. I didn't know anything about him, just the name. A while later, my mom's house got robbed, taking her laptop with it. I lost those 3 worlds. It didn't matter. I had my dad's PC and God knows how many single player worlds. I didn't play multiplayer.
My first survival world was as creepy as my first world. It wouldn't let me break blocks. I didn't like survival anyways, it was too complicated for a newb like me.
I created thousands of other singleplayer worlds, and I spent most of my time on this one world. I built a house and discover an island full of sheep.
Later on, I had found a cave. I was younger, and still a newb, so I fell in it. A shape greeted me at the entrance. The shadows from the cave made it hard to see what it was. It looked like a player. I blinked and it was a different creature; a sheep. Disturbed, I climbed out and ran far away. On that same world, I spotted a rail from a ravine. It was an abandoned mineshaft. I placed down a bed as a marker to remind myself where it was, and went back to my house to grab something. When I came back, the bed was gone, along with the mineshaft. I was pretty sure I headed in the right direction, but when I went to my house, I couldn't find it either. I was lost in the paradise I created.
I started expanding to multiplayer. I spent months on servers. I liked the company of the players and the mini-games that I could't even make myself. By the time I went back to singleplayer, I knew enough about Herobrine to give me nightmares. My friend told me this one story that I didn't quite believe, but now that I think about it, it could be true:
My friend was on an LAN world with her cousin when they discovered a cave. They mined until they got bored and emerged from their mineshaft. It was dark, and the forest was even darker. My friend noticed a pair of legs from behind a tree and thought it was the average zombie. She started to walk to the tree when a tip of a diamond pickaxe was shown besides the legs. "Herobrine!" my friend screamed as she and her cousin fled the forest.
I know, I know, it could be an 'Advanced Zombie' (A zombie that holds items) but it wasn't even 1.4, plus if it was, the pickaxe wouldn't be hanging from its hand.
Anyways, back to the story.
My friend climbed on top of a hill, her fingers throbbing from the constantly pressing W on her keyboard. But when she turned around, she was face to face with 'Him' and died. The death message being: "<name> was slain by Herobrine." (I don't want to share her username for reasons, do not ask why.)
She reported seeing him again on that world as he kept teleporting towards her (she was flying on creative.)
Other friends of mine gave me false info about seeing caves underground and diamond blocks with 'His' face on it. One friend saw him in the fog (her render distance was up) and Herobrine crashed her game.
Have I seen him? No, but I have seen things like this before:
-Random blocks missing from the ground in Super-flat
-Logging into a Super-flat world, trapped in a block
-Cave noises on Super-flat
-Strange things in the distance
-Tamed animals wandering off, even if in sitting position
The other day, I was on a Super-flat world, making a random build near a desert temple when I heard multiple explosions. No craters surrondedd me. 1.8 allowed Iron Golems to attack creepers, making them explode, but no villages I had encountered contained Iron Golems, and I did not place any TNT or spawn Iron Golems to attack the numorous Creepers wandering from the previous night. I didn't open up the hole leading to the tombs, and monsters don't spawn there anyways.
Everything, from other's witnesses to the 11th disc shows that something is in our world.
I don't know what it is, if it's Herobrine or null...
...all I can say is that he is The Shadow in our worlds...