I've heard about this being named "Herobrine."
People describes him as a spirit of a lost soul or a demon that was summoned by the same people who summoned the Wither.
But the day I saw Herobrine changed my life and how I saw things.

I was working in the mines one day and heard a slight hissing from behind me, I grabbed my blade and started slashing at random to hit the creeper, but- There was nothing there.
The hissing never returned but another sound came from the entrance of the mineshaft. One of my friends told me stories of her times in the nether and told me about a giant beast that cries through out the nether. I heard just that at the entrance.

I had begun to run to the entrance to see what monstrosity was emitting the sound.
Once I reached the entrance I saw it, a human?
He was staring flat at the dirt beneath him and breathing slowly. As I approached the man he started to lift his head up and examine me from bottom to top. As he did so I noticed something about his eyes, a strange glow loomed from them.
I looked around him to see if anyone else was around to see what I was seeing and also noticed that my vision was distorted all around the figure but upon looking back at him my vision corrected itself to focus only on him.

He opened his mouth and started a low pitched sound that was somewhat disturbing he stopped for a second and dashed at me, I fell to the ground screaming in complete terror. After a few minutes I opened my eyes and looked around, no one was there.
Not a soul.

A few nights later the same low pitched sound occurred outside my bedroom door.
I had forgotten all about the events that had happened in the mineshaft due to the nightmares I kept having so I didn't act in fear.

I grabbed some armor and a pickaxe and tiptoed toward the door, as I inched closer the door begun to creek open.
Dead in my tracks I stopped and peered at the door awaiting for something to lash out at me.
Minutes later I opened the door to find a piece of paper and a redstone torch right beside it.
The paper was blank but still I questioned who was here with me in my home.

Footsteps, growls, hissing and crying were I could hear for the next few days. It had gotten so intense that I began to pack my things and head towards the village a few miles north of me.
Only bad thing about the day I had left was the severe thunderstorm that swept in pouring harder and harder every step I took.
A mile or two later I stopped to take a break and eat something.
Blissfully enjoying my pork and apples I heard a whisper calling out my name from beyond the shrouded darkness.

A single redstone torch lit the darkness that consumed it. I struck my flint and steel lighting a torch and begun to head toward it.
Another note laid before the redstone torch but this one wasn't blank.
Two simple words were written onto the paper stating "Go Home."
I ignored the paper and looked around my surroundings and noticed a eight glowing red eyes that lurked in the darkness. I grabbed my sword readying to attack the beast.
It never attacked, only stared into my direction.
Another sound approached me from behind, footsteps of a human. I turned slowly to see whom or what was directly behind me.
When I fully turned I saw one of the villagers, arms crossed and staring at me. My heart started to calm down from the terror that was building inside.

The villager asked me if I was lost and if I needed help with something. I responded to him asking to escort me to his village.
before I left I looked back at the torch but it was gone.
I ignored it once again and started walking.

when we arrived at the village we went to the church to talk to the priest about some "things" I've experienced.
I had told him about all my weird experienced and all the sounds and nightmares I've had.

The priest told me about the mythos of Herobrine.
He also told me to stay with the village for further protection from it's wrath.