I will tell you about my horrible encounter. It might give you nightmares.
I was playing Minecraft 1.8 when I saw a world appear. It was called 'Paradise' I went on the world, it was normal at first but I came to a village to trade but the villagers kept attacking me. I built a small wooden house. I then went mining. My mine was so deep I couldn't see a thing. Luckily I had some torches. I saw a deep hole with ladders so I went inside. First I thought it was part of the update. I explored this cave. It had redstone torches all over. I found some diamonds so I had to mine iron to get them. I mined the diamonds. I saw a chest appear with some Diamonds. I took them. I was about to go home when I saw my little house on fire. I ran to the burning house and guess who I saw....
....Yes I saw the Secret Watcher! I thought The Secret Watcher was fake but it turns out it was real. I was on singleplayer too. Here he was with blood and a ripped suit. He put down a sign saying "YOU'LL PAY FOR THIS". The game crashes too. I saw the title screen saying "YOU ARE MY NEWEST VICTIM". I deleted Minecraft and downloaded it again.
If that wasn't enough proof then look at my friends encounter.
My friends encounter
This encounter is only short but its still quite creepy.
My friend played on his Minecraft and went on his world. But everything was on fire. I was watching him at the time. He saw a sign saying " I have scared your little friend". Then he saw him, the secret watcher. The game then crashes.