The Ender Dragon can pass through blocks, this happened when I got it down to low health, after that, I was inside the obsidian tower in The End, it managed to hit me while I was inside it, after that, I went down to the surface and dug tunnels, I went down about 30 blocks below and 50 blocks to the sides as fast as I could, but it kept damaging me even while I was digging to escape from it. That's when it killed me and I lost my 130-day Hardcore World.