I was gathering supplies far from home, and found myself in the midst of a sickly-looking forest, where I found a pit with walls of unbreakable black stone. I started descending the pit, scaling narrow ledges on the wall, when some gigantic flying parasite native to the area came in after me. I missed it with my musket, and it swooped down and knocked me from the ledge. I fell through utter darkness for a while, and was lucky enough not to scrape against the walls before landing in a dark pool.
I climbed my way out of the pool, pulled out a waterproof glowstick, and found myself in an expansive cave system deep underground. I was pretty injured and low on musket shot, but I had found a big haul earlier, which included an enchanted mirror that could bring me back home if I wanted... so I decided to press on for a little bit. Might as well see what the cave was all about, see if there was any loot to be had. If things got dangerous, I'd just use the Magic Mirror to whisk me away in time. I went along the cave for a few minutes, through pools of water filled with beautiful iridescent jellyfish, and past what might have been stalactites... though in the darkness, they looked almost like tumors with sharp teeth growing from the floor. Ahead of me, the cavern thinned down to a point, but near the end I saw a container made of gold. Treasure!
Just as I got close enough to open it, a screeching, bestial roar echoed through the caves... not from down the tunnel behind me as expected, but
through the cavern walls ahead of me. I didn't recognize it, but I knew it was a warning. The treasure forgotten, I start running back the way I came, fumbling through my gear for the Magic Mirror. As I stumbled back into the jellyfish pool, a gigantic skull with blazing eyes phases through the wall, flying like a phantom straight at me. I keep running, splashing uselessly through the pool, as I pull out the mirror, and hold it up to my face. As its magic started to take effect to whisk me from that hellhole, the phantom skull hit, killing me instantly.
First play-through of Terraria. Apparently the corrupted cavern I was in butted up against a Dungeon, and the Dungeon Guardian was pissed that I got too close. Is dealing 9,000 damage in one hit, in a game where your maximum HP is 400, really necessary?