I decided to reclaim a world-gem ruin. Things went pretty well to start with. The ancient dwarves had dug their spiral tunnel down through the first layer of caverns and set things up below them. This included walling off the caverns as they dug through them. I decided that this was cowardice on their part and knocked down part of the wall so we could use the caverns' resources; I also dug a well down to the lower cavern level to supply the hospital I was planning.
Unfortunately it was while I was doing this that the ancient dwarven wisdom became more apparent. A giant pterosaur with a single eye, a shell, and fiery breath appeared in the caverns and shot straight up the well hole. My dwarves were utterly powerless against it. The main problem was that they simply had no defence whatsoever against the breath and died like flies caught in a flamethrower. The beast did wander off into more remote sections of the fort, and I tried to wall it in, but the attempt failed.
I was down to three incapacitated dwarves slowly dying in their bedrooms and a single child wandering around the hallways, when a wave of migrants appeared. Here, I thought, I had a chance. No such luck: the beast erupted out of the fort's entrance into the enormous pagoda that the ancient dwarves had built over it, and set the forest on fire. Those migrants didn't last long, and neither did the few pitiful survivors indoors.
I decided to reclaim the fortress. Several attempts were all fruitless as even dwarves equipped with shields were terrified and scattered before the beast's onslaught, allowing it to pick them off easily before they'd even entered the pagoda, let alone made their way into what remained of the great hall. I decided to start an adventurer and come and kill it myself. Unfortunately I found that as soon as I arrived at the site the lag went through the roof. It had been fine for the entire adventure, but the moment I set foot on site there was a two-second delay for every step. I don't know why, given that there wasn't a single living soul in the entire place. But my companions were understating things when they commented on how horrific the destruction was.
I'm in the middle of exploring the fort, trying to find the forgotten beast, but it is unplayably slow. Which is a pity, as I'd really like to squash this thing and revive the fort.
Also, the ghosts are surprisingly chatty.