The adventure I have in my head is a situation where a party has to go to a structure and retrieve something. A quest more or less. What the party doesn't know is that the structure in question is possessed by demons.
What Daemons AreI would say that daemons are spiritual creatures that reside in the Astral Plane. I think that in most cases, greater daemons prefer possessing a person because they get more out of it. If deamons eat or "consume" anything, it would be the pain and suffering of people and maybe even animals.. That could be the driving force that motivates daemons to persistently enter the Material Plane so as to get the "food" they need from us.
I think in most cases, a greater daemon would get more food by possessing a structure and feeding off of the fear, pain and suffering of the entire group of people within. More people terrorized means more food.
Possessing a person might provide less quantity but perhaps a higher quality of sustenance for them. So, a greater daemon who has not fed for a long while might possess a structure first in order to get enough sustenance to build their energy. After that has been accomplished, the greater daemon might prefer to possess people in order to satisfy it's "sweet tooth" or craving, so to speak.
Lesser daemons get what they can get. While having the same cravings as a greater daemon, they do not have the strength to be choosy. If they are lucky enough to possess a weak person, hey, they hit the lottery. Lesser daemons are more likely to possess animals and small structures. It might require more than one to possess a larger, multi-level structure where the larger structures are easily possessed by greater daemons.
What Daemons Do
What's the difference between a structure being possessed and being haunted? A house that is haunted is said to have a ghost or phantoms, etc lurking within that are doing whatever they can to keep people away and preserve the solitude.
A house that is possessed is literally the new body for a Daemon . Think "Amityville Horror". The house itself moves it's parts on its own. While a ghost haunting might be in the same room with a person and slam a door or a window the people are in, the possessed house can slam any door or window or all the doors and windows simultaneously.
That's because the ghost has to move to each door or window to shut it but to the Daemon possessing the house, opening the doors and windows is the same as you or I flexing our fingers.
Ok so now we have a possessed structure in which at least one daemon has taken possession of. Is it a lesser daemon or a greater daemon? Could be either. Which is more likely? I'd say in this particular case, it is a greater daemon.
I am fairly certain that daemons of any sort do not possess the faculties to even feel "fear" or to be afraid of any other creature. They are not afraid of deities or devils or Planars or anything really. It's not even that lesser daemons are afraid of greater daemons. They just know their place in the order of daemons.
There are likely innumerable lesser daemons but a limited number of greater daemons. Greater daemons are named when they reach their full strength and have been identified by sentient beings they have encountered and named by. Usually by those in temples and churches that regularly battle daemons and follow or track their activities.
Daemons, especially greater daemons, never forget losing a battle or who they lost to. They are sore losers who will track down and re-challenge the person who defeated them again and again until they finally defeat them. Such defeat is usually not a good thing for that person who will be made to suffer for as long and as painfully as the daemon is able to make possible.
ExorcismClerics and paladins who specialize in exorcising or defeating daemons should beware they are likely gaining lifetime enemies who will perpetually pursue them in doing so.
For GM's/DMs, an exorcism initiated by a Cleric essentially casting an Exorcism spell. The Exorcism spell, for those unfamiliar with it, is not foolproof. It is begun with a base chance of success, which is left somewhat arbitrary in the books. I would suggest though that once a daemon has effectively possessed a person, an animal or a structure, it has the upper hand in staying so established. I also think that the morale of the victim should be checked and a modifier based on that be included. The lower the morale of the victim, the bigger the modifier to reduce that base chance of the exorcism spell succeeding. If we say that a lesser daemon is possessing, the minimum, un-modified base chance of success a Cleric has would be 50%. Modifiers based on victim morale, cleric morale, differences between the Cleric and the daemon's levels will then affect that base chance. If a greater daemon is the possessor, I would suggest a lower unmodified base chance between 30 and 50% is warranted. If the Cleric rolls within their modified chance, they have succeed. if the Cleric rolls above the modified chance, the spell is not broken, but the demon then takes a turn to attack. It goes back and forth like this until the Cleric is interrupted in the exorcism somehow. The Cleric can take damage during the process and can be made unconscious or killed if they lose the requisite HP, thus also ending the exorcism.
The level and HP of the Cleric or Paladin performing an Exorcism should determine (according to the Player's decision) how often they attempt to cast Exorcism. Do they have enough HP left to do it more than once a day? Is there a second Cleric present to assist and heal the exorcist if need be? Lots of things for Players to consider before beginning an exorcism.
Daemons gain or improve their health every round by 1-4 HP which is the physical/mental/spiritual damage they inflict on the possessed. That is if they are possessing a person or animal. If they possess a structure, they can only gain HP back at the rate of 4 hp/day with no people inside or taking 1-4 HP from each person they cause direct damage to.
Surviving a deamon possession is no picnic and the GM is encouraged to check the Insanity/mental Illness tables after they are freed from possession. Don't forget that from all the physical damage inflicted during the possession, there will be great chance of diseases and maladies due to open sores, cuts, scrapes, scratches, impalements, broken bones, etc....
Anyone witnessing said events of someone else being possessed may warrant a check to the insanity table as well.
Clerics losing an exorcism face being possessed themselves. They might be killed outright or driven insane (check the table). Daemons will always try to possess a losing Cleric to punish them and savor every drop of misery by torturing the cleric mentally.
The Faith score mentioned in the Demons/Daemons post before this could easily be affected by morale and morale of the cleric should be checked regularly during the exorcism. Daemons are relentless even in the face of losing so morale doesn't really affect them. They are essentially insane to begin with.
The Faith score of the possessed should also be checked regularly and can impact the outcome. Someone with a high morale due to seeing a cleric arrive to help could give the daemon a lower chance on their attack roll due to the increased resistance of the victim. Consequently, a low morale check could make it easier for the daemon and give a bonus to the attack roll.
Anything Other Than An Exorcism
Because daemons can possess structures and animals as well, an exorcism may not be what is warranted. This is where Paladins make excellent adversaries to daemons. Devising ways to destroy structures and kill possessed animals in a way that ends the daemon's time in the Material Plane is right up their alley.
Paladins can carry Holy Swords and use them to their full potential. Using a Holy Sword on an animal that is possessed or a re-animated corpse (a sort of a ghast for all intents and purposes) kills the animal or ghast and removes any possibility for the daemon to "jump" to another body.
The one area in which a Paladin can be out-matched is when an exorcism is requested or demanded. A very high level Paladin actually can perform an exorcism as at higher levels, they can cast Cleric spells. For example. It takes a 15th level Paladin to be able to cast a 4th level Cleric spell, which "Exorcism" is.