I found a large cave that runs through quite a bit of underground area. I was mining coal and iron for about an hour and on my way back to the entrance I started getting hit by an invisible enemy. I tried to fight back but I couldn’t attack it in any direction. I was killed and it listed that I was killed by a zombie. I immediately went back to that location to get my dropped loot. I picked up a shield to take with me on my way there. As soon as I Came to where I was killed and picked up my loot I was attacked again. I backed into a corridor I had dug to where I found the cave which was one block wide by two blocks high. It continued to attack my shield and produce damage. (I heard no zombie sounds during any attack) I swung with my diamond sword and then I tried a iron and stone as well. None of these seemed to phase this beast. It eventually destroyed my shield and quickly beat through my health and diamond armor. I have to say that was the most powerful zombie I have even been hit by. It took 2 hearts per hit every 3 seconds or so. Sometimes it was hitting me even when I was sprinting away. I was able to get my loot and sprint away fast enough to block him in. If i took the blocks away he began to attack again. I got him trapped and saved and quit the game. When I came back I couldn’t find him again. About 15 minutes into exploring the cave it happened again and I had to repeat this method to get rid of him. It happened again above ground on a hill near my spawn point a day later. I was able to escape to my shelter and restart the game. This is really discouraging.