An Entity from the Ocean

One day, I opened Minecraft and created a single player world. I named it "Hello", and the seed was "Goodbye". I turned cheats on, I put it on easy, and I allowed the bonus chest. Nothing too interesting. I created the world and waited for it to load. I immediately spawned on a 1-block wide riverbank. Next to me, on the left, was the biggest river I'd ever seen. There was a small cross shaped island in the middle, but I took no notice of it. To my right, was the bonus chest. I opened it. There was a iron sword, with odd blue textures on it. I picked it up. I was intrigued, because you never get iron in bonus chests. Only stone tools, wooden tools, basic foods, some wooden blocks and planks, and saplings. I also saw a book, so I put it in my hotbar. I grabbed the other normal items and decided to go into the forest beyond. I was mining a birch tree when I saw a small creek. A blue creature with one red eye with a blue pupil and one white eye with no pupil emerged. It had some dark blue, sloppy lines running down it, so it looked like water. I went into my inventory and got out the sword from the bonus chest. I saw a lapis lazuli block in the top right slot. I hovered my mouse over it and it said "Return my knife, or lose your life." Haha, I thought, that rhymes. But I should give the knife to it... As if reading my mind, the sword disappeared out of my inventory and when I exited my inventory the blue entity was holding it. A weird chat bubble appeared above its head, where a nametag should be, that said: "I shall return to my domain. Do not follow or you'll be in pain." I wondered why the entity had a strange rhyming speech pattern, but I watched it jump into the water. The usual water sounds weren't played. "Curiosity killed the cat," I thought. "But I'm not a cat, am I?" I plunged in after it. I saw it ahead of me. I followed it, and it swam to a huge ocean. It swam faster than anything I've ever seen. We eventually entered a sea latern tunnel. I followed, crouching. The entity was in no hurry, and was a slow walker, so I was able to keep up. We eventually entered a prismarine palace. Fish were swimming. ''Fish? ''Fish aren't in Minecraft! At least,  not yet... The entity hohopped onto a lapis throne and turned around, staring at me. The chat bubble came back, and said "You!" ''In the flesh. I typed. ''A round glass structure dsurrounded me and filled with water. please, no! I typed. I tapped escape, but no screen came up. Then Minecraft crashed. My computer shut down. I never entered that world again.