First page, 5th reply, In Soviet Dorfland, HFS mine for YOU!
first page, 6th reply, dorf goes nutters and gets addicted to HFS wall the way people do to every drug and eventually dies for it. Chance of going berserk
First page, eighth reply, More, tougher, diverse, demons with their own pets. Multiple Sieges.
Third page, excorcisms.
All of these combined would make for one hell of a time with the HFS, without requiring any of that "TOO DEEP" that apparently used to boot you from your fortress (does it still?)
Demons that sneak out and kidnap people would be interesting, especially if it happened unannounced except in the thoughts of dwarves that knew them like "idiotstick has been fine lately. He was unnerved by the disappearance of an acquaintance lately." Perhaps ones that go insane in the manner described in reply #6 could attempt to go there themselves if it's open, coming and going at random, becoming stranger and stranger, maybe coming back in a berserk rage at one point.
The suggestion of being outright controlled by demons is also pretty good.
If you think about it there's not much difference between demon and dwarf nobles...
Another
interestingscary idea would be a tag that allowed them to butcher and use any creature while it's still alive, maybe a seperate one for cannibalistic butchering, and some value in the creatures' raw would prevent it from caring if it were the victim of such.. This could be used on demons and the like to let them mutilate and eat each other, their victims, maybe themselves? The demons would need a way to regenerate body parts without resting though. (S)TDs, now ripping your arm off and eating it without killing you first! This could also happen to the ones that go insane due to the HFS
With all that, you'd have to also make them fairly inactive so they don't slaughter you outright, like allowing only one tunneler, kidnapper, and insane cultist at any time, apart from the sieges, then would be an appropriate time to let them throw absolutely everything at you (still only one of those three specials though. Exorcists, priests, and healers could be employed by the player along with the army to neutralize these.
I at least would be happy with this.
Once the HFS is breached, you might also see weird monsters either appearing from who knows where, or being somehow produced/mutated from existing animals by the clowns.
You could also get into various "bad signs": comets, blood moons, prophecies, strange births (again possibly leading to weird monsters appearing), maybe strange and/or famous deaths, fell moods, and the like.
This is awesome too, letting creatures above ground, newborns, and such become monsters occasionally and attack you. And the incorporation of the astrological suggestions thread could give this some neat effects and some predictability on the part of when things are more likely to occur.