My latest fort, The Dungeon of Dumplings, was my longest-running, wealthiest, most adventure-ridden and catastrophe-stricken fort yet. About six years in I'd decided to build my first-ever pump stack to see if I might be able to construct a magma moat, "waterfall" and specialised smelting reservoir.
Windmills were constructed and walled off above a system of connected gears, and the shaft meant to house the pump stack had largely been mined out. Unfortunately, despite having played DF on and off for years, I hadn't seen enough sieges to realise that wealth attracts them, so my fort was poorly defended and intricately engraved. The pump stack ended up being postponed for four years or so while I scraped through the inevitable and relentless goblin onslaught and set up a military of any worth.
Eventually I found that my military had become tough enough to take on minotaurs, squads of goblin riders and the like, so I decided to get back to work on the stack. Only the last couple of floors around the middle of the stack still needed mining, so the dwarves got to it, only to reveal that an open expanse on the third cavern layer had been wedged in the middle of the stack's design the entire time.
Seeing as a decade or so had passed since my fort was founded, my vampire King had died in the caverns long ago, and insects still picked at his marrow somewhere in those depths. Those layers had been sealed off immediately afterward, but somewhere in the region of eight forgotten beasts had accumulated down there over the years. Now that the layer had been breached again, my military immediately disregarded their burrow assignment and instead charged heartily into the subterranean labyrinths in search of precious, precious bolts.
Most of the forgotten beasts were fairly run-of-the-mill, and none sounded deeply threatening, but for one: Tig, an enormous earthworm made of kunzite, a type of gem, and exuding a noxious gas. I wasn't sure I could crack kunzite, and hoped that Tig would be satisfied to wallow in the thick pools he'd been spotted in.
I'd never attempted to control the caverns before, and my recent martial victories had made me brave, so I decided to remove the burrow restriction, let my greedy dwarves run amok and if they wound up impaled on each other then be it on their own heads. I constructed walls and doors before my new breach point to dwarven the place up a bit, and awaited the approaching battle-wails of the caverns' terrible promise.
Every few minutes, a new forgotten beast charged toward the breach with its utmost fury, and was put to death with deft ease. The outlook was surprisingly good, and although we were under perpetual siege, we now had as much forgotten beast meat, bone and silk as we could eat, three cavern layers of fresh water and native gold, and we'd finally put the vampire King to rest in his stupendous tomb. As the flow of beasts steadily reduced to nothing, I ordered the last cavern walls erected, the doors replaced and the military back to the fort proper.
The final beast, Tig, came barrelling down from an unsurveyed hill North of the breach, skimmed straight over the cavern wall in a shimmering frenzy of purple gas and immediately set about savaging every dwarf within reach of the entrance. I'd hoped he'd gone home. I set the military on him immediately, and without a second thought they barged toward the monster from both sides. The area was engulfed in great gusts of excreted gas behind which the foray was soon hidden, but V seemed to show that the gas didn't have any immediate effect aside from numbness.
After a minute or so, the smog began to clear slightly, revealing the smashed carcass of the kunzite earthworm and a few incapacitated dwarven victors. My medics dragged them back up the pump shaft to rest in fine hospitals, but as I followed them I noticed that the worm's gas seemed to have risen from the caverns all the way up the shaft and through the rest of the fort.
Eventually I realised that this was the dreaded necrotic gas, that half my military was rotting alive in their beds, and that the purple mist wafting throughout my fort was the miasmatic stench of the warriors' rotten bodies being dragged around. One of the surgeons braved the purple fog of his hospital to diagnose one of them, finding that every part of his body, inside and out, was in an advanced stage of rot, but had nevertheless set out to save his life by scheduling major surgery on every piece of the guy from eyeballs to innards. Upon checking the diagnosed dwarf more thoroughly, I discovered that he was also the very macedwarf who had struck the killing blow to Tig.
Years later, after most of the gas victims had suffocated in their own stinking mire, I happened to notice one of my old masons wandering around. I hadn't seen him in ages, and had assumed he was dead, so I looked him over. He turned out to have been another victim of the gas attack, but the surgeon had actually gone ahead and done the daft amount of surgery necessary to save him, presumably over the course of 2-3 years. He had full-body scarring, permanently mutilated, but he was alive, he could finally walk around, and now could be a mason again. I was so pleased.
Later in the same fort, one dwarf lying in hospital with a broken leg woke up and decided he was going to make an artifact, leg be damned. He commenced to slowly hobble back and forth between the sprawling stockpile layer and the tannery, gathering one bone at a time from the refuse pile in the corner and dragging it back as the other dwarves milled past his open agony in polite ignorance. It was painful to watch the poor fool, every time he'd finally make it back to the workshop, sweating and groaning, he'd immediately set out again for just one more bone. He almost passed out from the effort.
I spent ages trying desperately to get this legend what he needed, even weaving silk cloth from forgotten beast webs just for him, but he insisted on yarn thread, and my other dwarves absolutely refused to shear the llama. In the end, after I'd given up, locked the tannery door and hadn't heard from him for a while, I tried to find him to see if he was still sane, but he wasn't on the units list. I checked the locked tannery, and nobody was there. He'd completely disappeared.