Last year and a few months ago. I was playing minecraft on a survival world. I was gathering resources and items until I saw something in the distance. I thought at first "Okay it's just a enderman" because I have been playing for 2 nights in minecraft. Until I saw the same thing spawn again.
At first I thought it might be a rendering glitch with a cow because usaully those happen when you're on a bad pc. But soon it turned to me and it had purple eyes. When it exactly stared 100% at me, it disappeared. I started to think I was tripping so I went down into a cave to get resources.

I was mining iron until I heard a unsettling sound. "That was just a cave sound so don't be scared." I said since it was. But 6 minutes later I heard another sound but this sound isn't minecraft cave sound. It was like scratching and scrapping and roaring like a minecraft warden with disk 11 plus 13. However that wasn't when the minecraft warden came out. Soon after 2 minutes I heard the sound again. But instead of a warden like roar, it sounded more like a distorted version of the ender dragon, with the old sounds of TNT.

Nothing happened until 4 hours later (Yes, I loved the game so much back then, that I played for hours). Until my inventory items swapped data with different items even more than the max item limit(64). My sword was like swapping from stone(Original) to a stack of TNT to 24 diamonds, 125 dirt,
21 wooden swords, 2 pickaxes, 6 beds, 3 doors, Fully enchanted max netherite sword,
Enchanted gold ingots, Then it repeated back to the order it was with the chat spamming PTDHAJOPUNFVB. Soon I saw more eyes which looked more liked the eyes of the ender dragon.
I knew it was going to disappear so I took a screenshot of it. Video on https://www.youtube.com/watch?v=rIcmCLbLXGg.
After 30 minutes of chaos it ended.

I exited the cave and ran back to my house. I hid in my house and left the world for 3 days. I was wondering what was PTDHAJOPUNFVB. So I tried to find meaning for it and it turns out that PTDHAJOPUNFVB was not a word nor a name for anything but a Caesar Cipher. I translated the Cipher and it said IMWATCHINGYOU which was squished together characters for I'M WATCHING YOU. I went back on the world to see what was up and my house was on fire!

I quickly ran out of my house and the chat said IYEMKXDOCMKZOWO which was a Caesar Cipher for YOUCANTESCAPEME which again was
YOU CAN'T ESCAPE ME. Then I got a jumpscare with the eyes of that same shadow enderdragon with the words I AM THE DSLOZH PYOPCOCLRZY! Which was again the cipher for I AM THE SHADOW ENDERDRAGON! Then the game crashed.

I got a popup window on my computer which said "I'm at you're home! =D". Then I heard a knock at the window and thought "That might have been a bat hitting the window". When I went to check I saw 2 purple glowing eyes like the same shadow ender dragon. I ran out of my room and hid in the closet. When it was day I went back into my room and deleted the world files and now have my blinds closed during night.

Still to this vary day. I still couldn't figure out what that thing outside was. But at least I know that the entity is called the "Shadow Enderdragon"