Demonhunter was suggested as a Demonologist noble two pages back, as was an invisible murder beast. The above duplicate suggestions, though, have a slightly more robust spin. I don't know if
defiling the entrance to another hell-dimension is such a good idea, and it certainly wouldn't stop the onslaught.
Having just recently come into the joys of the Arkham Horror board game, I think it would be interesting to have the option of
resealing the pits somehow. You'd still have to kill the demons pouring out before being able to do it. If the onslaught is endless, then a mechanic that puts the siege on pause would be necessary to allow you time to seal the pits. Like, say there is a demon holiday where the denizens of The Eternal Plane of Fire are not allowed to slaughter mortals for a week in penance for whatever, or perhaps for one week a year building up to the first day of spring, the pilgrimage of the Forest Spirit from the Realm of the Dead to the Mortal Plane makes inter-dimensional travel impossible for evil forces.
Just something that gives your priests/demonologists/alchemists/what-have-you enough time to get in there and reseal the pits. Assuming adamantine is the only thing that can keep the demons caged, I would assume it would be involved in resealing.
Okay, I've worked this over in my head for a bit, and this is what I got. In case you need to be warned this far into this thread HFS spoilers to follow.
These ideas are kept fairly mythology neutral, where possible, but there are going to be some sub-suggestions that come under countermeasures that some of you may find objectionable. We already know, you don't have to tell us.
The smash/burn/kill/defile demons in the pits and the horrible nightmare evil spirit creatures are the same beasties. If you kill the demons, the nightmares stop. This allows the dwarves to fight the demons on their own terms, in the traditional DF manner.
In fact, once you open the pits and the legions start storming out, the nightmares and whatever get put on hold in favor of a more direct application of death and horror. The psychic torments were just a stopgap that the demons applied, because they couldn't attack the dwarves in the more visceral fashion that they prefer. (This one does assume that the demons are unable or unwilling to tunnel to the dwarves, but still aware that they are "out there".)
The nightmares are whatnot should be an inconvenience at most.They should take a couple of dwarves here and there, through death and madness, and increase the chances of a spiral, but they shouldn't be a fortress ending cataclysm in their own right. They should mostly be about fear and atmosphere, rather than any sort of mechanical challenge.
From a game-play standpoint, the battle with the physical manifestations of evil is the climax, and the haunting stage is just the build-up that adds anticipation and flavor to that moment. In that regard they should slowly grow more intense over time, to push that sense of building drama, and eventually encourage the players into a confrontation with the horror that lives next door.
The nightmares should probably start with the first square of admantium mined(or even exposed), and row faster the more squares you mine. I know this seems harsh to those of you who are used to mining isolated veins with complete impunity, but I think that this adds the sort of enjoyable tension and "one more square" mentality that digging too deeply and too greedily should be about. Again, the horrors shouldn't be as much of a threat as a full on glowing pit scenario.
An experienced HFS miner might even be able to guess how many demons are in the pit below, from the level of horrors he experiences. This might seem like a bug, but it lends itself to a nice "Ooh, this must be a big one."
Countermeasures:
You should never be able to eliminate the effects of the call of Pit'thulhu on your fortress, but there should be a number of ways to mitigate the effects.
Doctors, priests, philosophers and medics should all be able to train themselves in the art of recognizing the symptoms of demonic possession and oppression. People are going to come to them with these problems, and they need to be able to figure out what they are dealing with.
Since the demon will just start harassing a new victim, there's no point in burning them, but that doesn't mean that your dwarves know that. Mandating the burning of the possessed would be a flavorful thing for athourity figures to do. This might give dwarves a happy thought, because they feel that something is being done about the problem.
Likewise, even if amulets and priestly blessings don't do anything, they might give dwarves the courage to soldier on through these troubled times, without taking an axe to the guy next door, when he rearranges his furniture at 3 AM.
However, it would be cool if the various religions did have some means of invoking divine protection, or abjuring harmful spirits. They could provide you with physical objects, like blessed amulets shaped like the totem animal of your god, sacred unguents(like holy water, sacred perfumes and anointing oils), scrolls inscribed with quotes from prophets, and the finger bones of a famous archbishop, or they could give you something immaterial like a prayer to say before sleeping, and a verbal blessing of their own.
These could make the nightmares come less often, or at least keep you from turning your children into artifact mittens. Over time, the demon's hold on a dwarf grows stronger, and you need more warding to keep it out of his skull. Thankfully, your priests are getting plenty of practice, so they should be up to the task, unless one of them has an accident, or get's possessed himself.
Alternatively, the fortress might try to appease their diabolical neighbors with worship and offerings. These could stop the growth of the problem, or even cause a single demon to stop causing trouble, so long as the offerings continue. As these are the histories of geed and trouble, the demons should demand an ever increasing amount of tribute, lest they visit horrors upon you once again.
The demands could be made known through the ravings of the possessed, or the cult leaders might have some means of auguring the demons will, without being completely lost to madness and possession(though it should probably be an eventual side effect, if they keep on communing psychically with demons). Some might even pursue possession, for the power it would offer them over the other cultists.
More Fun for everyone!
I like, Felblood. You laid things out in a nice, reasonable manner. I agree entirely. I wonder what would happen if a priest is possessed...it should be more interesting than the lay dwarf getting possessed, more insidious. Like, the priest may start a cult and start converting dwarfs to do the evil bidding of a dark god. You don't notice or hear anything about it until you see four or five dwarves in a far-off corner somewhere "holding a meeting" dressed in hoods and cloaks. Later that room has "pidgeon remains" in it and blood splatter.
I am a little disturbed by the attitude that things in a fantasy game should be kept only in the realm of rational explanation or that the supernatural/paranormal has no place in DF. There is no problem with people preferring to play whatever way they desire, be it with brute force, wizards, philosophers/alchemists, or faith. But it is wrong to argue that the game should not include any of those methods as an
option to deal with a fundamentally 'extra-planar' threat based on the fact that it represents an homage or reference to the reality of how real humans deal with the unexplained. The point of a Tolkienesque fantasy is to experiment with the what-if of mythologies (that is, dead faiths) being the manner in which the universe actually works.
Unfortunately, most such universes get it wrong and put forth a mythology and supernatural mechanisms that make it so that faith or magic-based solutions are the only viable ones against the supernatural or even any remotely interesting threat of scale. It is a legitimate fear to have that our beloved game my go this way, and it would nerf our armies and traps via relativity if some totems and amulets were 10x more effective at thwarting the pits. D&D is a prime example: a 12th level wizard is many times more deadly than a 12th level fighter given the right spells and some ingenuity. At least at 15th level a cleric can
bring back the dead with little consequence (as far as the rules strictly go) and a wizard can deal 7d6 of lightning damage instantaneously. No one is suggesting this kind of misbalance. What is being suggested is that a full spectrum of methods for dealing with the pits should be effective in a balanced manner, allowing the
player to choose how they want to primarily deal with the situation. Amulets and totems can soothe your dwarf's tempers and fears, but so can tinctures and medicines made by your Alchemists, public display of slaughtered demons, confinement of the supernaturally insane, etc.
I've said it before: a game in which the gods have known birthdays only a few hundred years past, primordial demigods still roam the world and attack your fortress, and unscrupulous mining results in the opening of gateways into
the evil planes themselves cannot escape having a meaningful system of faith that goes beyond mere superstition.