There's a long tradition of combating, even defeating, ghosts. Sometimes the combat can be brutal and spectacular; it took twelve ecclesiastics to beat Black Vaughan down, for instance. Generally in these tales the objective is not to force the spirit to move on to a different place. The idea is to imprison it here, on earth. As a spirit, it can't be killed, and as it's malevolent, it can't be sent to heaven. The only option, short of somehow sending it down below, is to keep it here, but in such a way as to render it harmless. Shove it into a grandfather clock, throw it down a well, put it in a room and seal the room - however you care to do it, do it, and then forget about it.
Much like vampires, really. Except that vampires have a specific list of banes where ghosts have only one recorded weakness: prayer, especially when delivered by sanctified holy men.
Night's Black Agents is a more active setting than Bookhounds. The agents are expected to be capable, combative people. It's less about arcane knowledge and more about how many rounds you can put downrange.
I gave these guidelines for Bookhounds ghosts, or spirits of place:
- The truth of the haunting will probably never be known for certain, since most of the facts are unavailable.
- It cannot be dealt with in the same way as, say, an ordinary antagonist encounter. Ghouls, for example, can be shot, or bargained with. There is no way to communicate with a haunting of place, and probably no way to kill it.
- It has a great deal of power behind it, possibly magical power. That means other people besides the protagonists are going to be interested in it. That also means it could be very dangerous.
I'd modify them for Night's Black Agents, as follows:
- The truth of the haunting must be linked to the Vampire background. If vampires in your game are mutant creations of science, then ghosts should have a scientific background as well. A Satanic vampire game has Satanic ghosts, and so on.
- It cannot be dealt with in the same way as an ordinary antagonist encounter. Ghouls can be shot or bargained with, but ghosts don't have the same weaknesses. Bargaining may be possible, but difficult.
- It has power behind it, but that power is going to depend on the method of its creation. It ought never to be as powerful as, say, a Renfield, let alone a vampire. This isn't a major player; it's a mood piece, possibly even a booby trap.
- These ghosts can be defeated but probably not destroyed, in the same sense that vampires can be defeated, but can come back from the grave.
In NBA vampires come in four delicious favors: mutant, supernatural, damned, alien. What kind of ghost stories can be told with the same premise?
Mutant: Their markers are medical symptoms; their emphasis is infection. The ghost is a vampiric remnant, something that lingers in those areas where vampiric infection has occurred. Say the vampire attacks and kills someone; the ghost is what's left behind, and can be dealt with by cleansing the area in the same way crime scene cleaners deal with the aftermath of a bloody murder. It might be inhaled, or infects through contact with unprotected skin. It might be some fragment of memory from the victim - their daughter's first birthday party, say, which causes anyone infected by it to relive that day again, and again, and again. It might be something left behind by the killer, an eye infection that causes the victim to see, say, blood, whenever they look at, or are in the presence of, certain things. Say the murder victim was a blonde female teen. Now, every time the agent sees a blonde of about the same age and gender, the agent hallucinates blood. Dealt with by medicines, or injecting liquified Banes.
Supernatural: Their markers are strange superstitions, their emphasis hunger. This best fits the traditional ghost story, and is the best candidate for magical manipulation. If, through magic, an unruly ghost can be imprisoned in, say, a grandfather clock, then it can be used as a supernatural bomb. Send it to the target, and sit back and watch the fun. Casting the Runes is the prototype. It's never clear, in stories like these, whether the ghost is a human spirit or some kind of older, pagan thing. A semi or demi God, perhaps. Some remnant of, say, the Great God Pan. A ghost of this sort probably has limited intelligence and free will, and the older ones can be very dangerous. You don't survive several hundred, or thousand, years, without learning a trick or two. Dealt with through arcane rituals found in worm-eaten texts.
Damned: Their markers are holy symbols and spiritualism, their emphasis is seduction. These ghosts are the bargainers, the promise-makers, the succubi and incubi. They are likely to be demons in their own right, capable of possession. They have a great deal of power when they're linked with a mortal soul, much less so without a suitable host. The Exorcist is the best example. Hungry Ghosts work well in this paradigm too. Of course, exorcists come with their own baggage, and rising demand for their services probably indicates widespread despair; the world is burning, and it must be the Devil's fault. Dealt with through spiritual intervention.
Alien: Their markers are various uncanny effects; their emphasis is invasion. In this version ghosts might be the aftereffect of alien tech, or just the presence of aliens. Quatermass and the Pit is the best example. Deep beneath the earth the last Martians lie entombed, and wait for the day when they can take over, piggybacking on our minds to recreate Mars. Until their rocket is uncovered, they can only throw out psychic shocks and disturbances - which is why Hobbs Lane, the London street that is being dug up, has such a shocking reputation. The Devil lives there, they say. Sure enough, there have been strange sightings, eyewitness reports, and horrible scenes there since the beginning of recorded history. In this instance ghosts are almost a warning, the canary in the coal mine - for if you see them, you know this is a tainted place. Dealt with through avoidance, or some kind of prophylactic treatment. Tin foil hat, anyone?
The big takeaway, and the difference between these and the Bookhounds ghosts, is that they can be understood, challenged and defeated. It may require magic, or some kind of chemical cleansing, rather than a Glock, but the end result is the same.
Ghosts of this type should generally not be very powerful; that spot is reserved for the vampires. Their main function is to squick or mislead, not defeat or kill. Aberrance rating, except for particularly powerful entities like the Great God Pan, ought to be low; somewhere around 5. No Free powers, and some will be more common than others. Possession seems one of the most likely for Damned ghosts, for example.
The other thing to remember is, these ghosts can't be killed easily, but they can be diverted, imprisoned. Which means the agents can also weaponize them, if they can work out how to do that without getting caught in the blast radius. That's a story in and of itself. Figuring out how to do it is one thing; pulling it off in the heat of the moment, quite another.