I've been playing the same fortress for a while now, and after a few days of playing it, a tarantula like forgotten beast appeared. I wasn't particularly worried, as my staircase had pierced the cavern it was in through the ceiling. As I dug further down, I found a magma sea and began to set up a magma metal processing facility. Unfortunately, since the cavern obstructed my main staircase, I had to dig a tunnel to the east in order to continue a new downward staircase. My one mistake was not to cover over the old staircase. After a few more days, I start getting an "Urist McScaredypants cancels brew alcohol, interrupted by forgotten beast". I looked around and realized that the forgotten beast had come quite close to the end of the old staircase exposed to the cavern, and was scaring away half my dwarves further down into the ground, unable to get back to my main fortress. I tried all means of escape, but water from another underground cavern prevented me from digging a newer stairway. It looked as if I would have to sacrifice half my fortress to this forgotten beast, Ngogngo. After a little while of waiting, I notice something, Urist Matatis, one of my new crossbowdwarves had gotten stuck down there too. Suddenly I started getting new combat reports, it took me a few moments to understand exactly what was going on: Urist Matatis was firing at the abomination with my finest iron bolts through the staircase, quite a sight to behold! And so on that day he got his first kill: Ngogngo the forgotten beast, d. 7