So I just made a for to try some stuff out... I wanted to have a tower cap farm, magma workshops, and all the benefits of HFS so I used reveal to find where all the features were then reloaded and 'found' them all. I breached the HFS by dropping a shaft 1 square away and then carving a fortification into the chamber so as to not release the demons. After getting all that squared away I then went on to experiment with damming my underground river by dropping a block of stone from above (which worked well, but that's not part of this story), and then made the rather poor decision of recruiting all 7 of my dwarves into the military and sending them in to clean up the critters in the dried up channel of the river. Well I shortly found out that noob recruits are no match for a mob of lizard men and ended up losing all 7, so I thought 'no biggie, I'll just reclaim and carry on'...
Well that is where the fun began. When I embarked I immediately started getting 'demons from the deep' spam and my 70 odd dwarves started killing SoF's while taking heavy casualties. After a bit of this I was down to about 50 dwarves and was having no more demon spam, so I assumed that the demons were all dead so I started trying to clean up the mess and do what I could to deal with the tantruming dwarves running around. However not 30 seconds into my break I start getting the demon spam again and am shortly down to 15 or so dwarves so I stopped and had a look at the untis list and counted no less than 25 dead SoF's and 3 living...
My confusion stems from how they managed get out of the pit as it was still sealed with a fortification, and why did it seem like they were continually respawning? Is this normal when you try to reclaim a fortress where the HFS has been found, no matter if it was never actually breached with a navigable path?