So, if you didn't already know, HFS is accessed by the adamantium pillars that start some levels above the "Magma sea" -level and end in HFS. The *only* way to access HFS is to dig shafts of staircases in these adamantium pillars, as everything else will be "semi-molten rock" which is basically just undiggable rock. The HFS is immediately below this undiggable rock.
The HFS itself is (needless to say) so much fun. Right when you dig into it, the game produces a message:
And after that:
Then the fun begins! You take a look of what lies inside, and:
There is about 25 of them. Here are the types that appear in that flock, there are one or two different species that cannot fly and just hide in the bottom of the HFS, such as some "Eyeless Brute" which I didn't manage to get a screenshot of.
They all can fly, and move very fast. The miner who dug into there will surely die, unless you open the level up with an intentional cave-in (I don't know if it's possible). Most (if not all?) of them are magma and water resistant and can swim in both. The steam dudes pass through fortifications and such.
So, how to get rid of them, you ask? I have a plan, which I tested a moment ago and it worked: make a separate tunnel into the HFS that's not connected to the outside world. Then trap the baddies out of their own home, preferably into some kind of a prison. With some tuning you won't even get your dwarves killed in the process! I've got a movie but it's too large for DFMA so I need to see about that sometime :-D
How to preserver your dwarves, you ask? Simple.
Make a large hall, separated from the rest of the world sealing the suicide miner in, for the demons to go into. Fill it with bait animals, preferably place the animals in the end of mazes so it takes longer for the demons to get there. Make a maze on the main entrance to the hall, too, so that it'll take ages for them to get there.
When all of them are in the hall chasing after your animals (the hall must be sealed from your fortress, and preferably the outer world too if you wish to have any caravans or immigrants!), commence a dwarf to wall off the entrance back to their home, sealing all of them off into the separate area. Then you are free to go into the HFS with your living dwarves, IF you sealed yourself properly and aren't dead at this point.
The HFS will still probably contain one or two monsters, so be prepared to fight them off with something. They're quite strong, but I guess their strength relies heavily on numbers - a group of adamantium-armed skilled soldiers should dispatch one easily, whereas dispatching all the 25 spirits that fly up on you in the beginning will be very difficult.