WE ARE RATS ON A SINKING SHIP
Ghosts aren't normally visible when they're outside of ghost land. Being perfectly still and breathing slowly may relax the rules separating ghost land from the living world and you may be able to see a ghost. If you can see a ghost, it can see you. In ghost land, you can see a ghost like any other thing.
A ghost in the land of the living will find something to haunt and haunting causes the ghost to grow without limit. Larger ghosts are more powerful, dangerous, harder to banish. A ghost banished from the living world stops growing, but never shrink so each time they return, they're as large and as bad as when they left and then they continue to grow until banished again.
Ghosts all live in ghost land and ghost land is basically the planar/planetary version of a ghost. It haunts entire planets. There is no known way to extricate a haunted planet from its clutches.
Every so often, the rules delimiting ghost land and a living world are no longer able to keep distinct the world of the living and the enormous, writhing mass that is ghost land and all the people and gods of the living will be consumed. This has happened three times already.
It's about to happen again. Ghosts are pouring into our world. Some go to to ghost land to find some way to fix the imbalance and plug the leaks. Some go to revel and delight in this strange new world or just to have something to do before we all die. Cults worship certain ghosts (and are granted spells). Some go to earn glory and some go in the hopes of actually colonizing the place (especially the wealthy and powerful, who are paying gobs of money for a safe place of relative comfort and power in ghost land), finding some place safe for the living once our world is destroyed.
Urz-Uk the Hollow God says he knows a way to save the living and is gathering worshipers.
Many other gods have gone silent, perhaps already consumed by ghost land, perhaps having given up or killed themselves. The laws of nature on the living world are buckling under the systemic pressure of dying, absent gods and being slowly consumed by a world-eating alien creature.
It's unclear how long the living world has until it's consumed entirely.
GHOSTS DON'T CARE
In any event, ghosts are pretty much ignorant of what's going on in the real world outside of how it relates with their particular need, if it relates at all, and if they were once a living person, they don't remember it. It's a rare ghost that cares about the living world, it's an even rarer that might care if it were destroyed.
Ghosts tend to be oriented towards a single action or emotion and pursue their desire unceasingly. Their bodies are bent to it, their minds are made only for it. Usually, the desire is horrible. If a ghost is a ghost of pleasure it may desire nothing more than to fill its cavities with dirt and may try to do the same to you, not because it wants to hurt you, but because it needs every open place within you to be packed with dirt because that just feels so good. A ghost of beauty may clip from children certain fingers it found particularly objectively beautiful and give them to you; it will be greatly offended if you don't enjoy it's taste. Most ghosts serve a desire less sane/comprehensible.
It's uncertain as to why so few ghosts are in ghost land when so many of the living have died and why ghosts have little to no memory of once being alive. Those that do remember, though, don't really care; they aren't what they once were by and stretch of the imagination.
KILLING GHOSTS, HEXES, CURSES
Curses and hexes work this way: the witch, warlock, magic-user or cleric invites a ghost to haunt an object, location or person. The influence of the ghost causes some deleterious affect. Removing the curse simply severs the connection between ghost and whatever its haunting, and doesn't banish the ghost who is still free to roam the living world. Clerics know the rituals to banish ghosts. Most people don't know it works this way, including the people casting these curses and hexes.
As the ghost associated with a curse grows, the complexity associated with keeping its power in check grows as well. Eventually the curse that calls down early baldness on an unfaithful lover strikes hairless everyone in sight. Some curses are simply never invoked as whatever they once did, they now cause instantaneous death to all within range, including the invoker. Some bonemen will slip a too-potent curse into the spellbook of a rival to just this end.
Ghosts in our world can never be killed, just damaged enough that they lose their grip on this reality; "killed" ghosts float back up to ghost land in a puff or cloud of smoke that smells like burning rubber or electrical fires. Ghosts that are banished forget the last day they spent in the living world.
The parts of ghost land we can get to easily clearly correspond to places in our world; the terrain and buildings of the living appear in ghost world as well, albeit empty of every living, sentient thing (semi-intelligent constructs and the undead may appear, but only as swaying, empty shells of themselves.). There are other places in ghost land that are entirely novel. They may belong to another living world to which ghost land is attached or, in some cases, appear to belong to now-lost places, having fallen to war, overpopulation or having been swallowed by ghost land entirely.
It is, of course, possible, that some of these stranger places are simply one of the many places we've yet to discover in the living world.
The weather in ghost land mimics that of the real world, but for the tides and the winds. The moon in ghost land is incredibly close, close enough to touch much of the time and this causes incredible tidal reactions. The ghost land towns whose real world originals are located along shore lines may spend part of their day under water and often stink of rotting seaweed and have barnacles all over.
There are also the change winds that blow across most of space, the freezing cold breath of the north (like, literally freezes blood in the veins and ruptures hearts) and the boiling hot scald of the south (again, blood so hot it issues from you as steam). Not that the winds interfere in ghost land all that much; while they can utterly destroy an area miles and miles wide, ghost land simply reconstitutes itself in seconds.
Despite being a map of the real world, ghost land has problems with consistency. Sometimes the cup you left on the table in the real world is a crying wax tot in ghost land. Rooms in ghost land can be inversions of what they are in the real world and some things can forget they're meant to be inanimate objects (the door that becomes a mouth, the wall that wriggles and feels like skin). Sometimes the person who was standing next to you is over there and whatever is standing next to you isn't a person at all. After all, ghost land is, itself, a ghost.
Ghost land re-maps itself to the real world every morning. Buildings destroyed in ghost land grow back together to match the real world and similar alterations in the real world are adopted by ghost land. The enterprising and unscrupulous can duplicate things left in the real world using ghost land as a factory. There are factories in ghost land; ghost copies often come with creepy or terrible sideeffects (haunted by a ghost, is some kind of ghost, it's user never sleeps but travels to ghost land every night until she dies of exhaustion, possesses the user, melts into a relatively useless mushy pile of ghost guts at some point, poisons everyone you or the person you love, can only be sheathed in the body of something living, creates an endless supply of ravenous frogs, etc, etc).
The lost places in ghost land that no longer correspond to anything accumulate glitches and deformations over time. They slowly stiffen and petrify. Similarly, ghosts that grow old enough they forget their purpose eventually harden and petrify. There are whole forests of these things in ghost land and many tourists and adventurers seek these out as ghosts tend to steer clear of these areas (and petrified ghosts are great for making stuff).
The stars seen from ghost land are entirely alien. Astronomers are mapping them and may need your protection for which they will pay you in maps.
cities, towns and the rest of civilization in ghost land are copies of the ones in the real world. The precise nature of the copy varies; like, the city of A____ in the real world is mirrored (ie, left is right, right is left) the "reverse city of A". In ghost land, there are upside-down cities, reverse cities, mirrored cities, light-less cities, under-water cities, etc. Clever thieves with a bit of magic or the permission of a powerful ghost may use the strange geometries and inconsistencies of a ghost city to get into places otherwise impenetrable in the real world (for example, there may be a door into a vault in ghost land where no door exists in the real world; enter the door in ghost land, then use a modified rope trick to climb through the ghost land vault's ceiling and arrive in the vault in the real world). Bonemen, witches and the ordum malus are often employed by the rich and powerful in the real world to prevent or limit such incursions.
Ghost land can be surprisingly empty of ghosts. Travellers can walk for days and see only a few. Most cluster around certain areas and generally prefer places that are less frequented, dark, and full of narrow little places to hide or hunt.
these are some of the more common of the ghosts (ie, there are more than one of these). Communicating with these ghosts is generally impossible (they aren't interested in communication).
Snatchers, ticklers, grabbers, pokers all hide in the shadows and try to interfere with anyone walking by. If they grab hold of you, they may not let go and try to possess you (see Possession), strangle you to death, wring one of your appendages or squeeze your middle until they've squeezed you into separate parts, twist your limbs off like sausage makers, rub and tickle you relentlessly until their fingers eventually slipping past ruined, chafed skin to the soft and bony insides, or else their touch is lethal or imparts some doomed fate. Their lightest touch leaves a dark bruise.
Wax tots are made of wax and mangy hair and dull, rotten old people teeth and have to be melted or breast
fed or their cries will wake their parents. Wax tots are whispered about by mothers and wetnurses (you must feed them or you'll be found next morning, your body slightly doughy, a bit runny and your hair burning steadily).
peeping toms, evil eyes are generally indistinct, though they may have some kind of face and will always have wide, penetrating, unblinking eyes. The relationship between the peeping tom and its victim is unknown, may be entirely at random or may be tied to some fetish or ideal the peeping tom seeks but once the victim stares into the eyes of the peeping tom, they are doomed and will some days later walk into a bit of shadow and vanish.
Reeds are incredibly narrow, stink of rotting marsh grasses and spend most of their time with their fingers and toes buried in swamp water, staring and moaning at the deeper parts of the water. Won't attack unless you interfere with them or their swamp (including breaking their line of vision with the water).
Dirtyfingers will try to stuff you full of the rancid mud they pull from their orifices. Often found in areas where soft dirt is plentiful and uncontested where they roll about and shove dirt into their many opening and roll their eyes in ecstasy. Usually around wetlands or in forest. As they hold you down and stuff mud into your mouth they will whisper into your ears from clogged mouths, "Excellent, excellent."
The hive are sentient flies, albeit with ragged fingernails clippings for legs. They delight in dancing across the nape of your neck. They have a single intelligence and they are building something inside the hollowed head of a mad titan in the reverse city of Lahz'anshleez.
Envies are made of eyes and arms and that ignore all attacks in an effort to get to their prize (usually are shaped roughly like writhing balls, but not always). They will grab an attacker's sword, shearing off their own fingers or impaling the sword deep within their central cluster, just so they can grab their attacker's arms and legs, pull them to the ground, clamber atop them, unbuckle their armor and then begin removing parts from their victim which they try to jam onto themselves (they just stick them there, so the eyes, tongue, jaws, genitals, liver, what have you simply falls away, which is highly frustrating to the envy).
lantern heads live in forests and congregate around bigger trees. None is taller than three feet, most are much smaller. Some have heads like lanterns, others like a carved stone face out of whose eyes, nose, mouth and ears issue light. Usually are shy, but they know where the scary guys in the forest are. these feature heavily in the stories of the Ent.
weavers look just like spiders and are no larger than a quarter, but they remake reality, slowly reworking it with their spinnerets as they move. weavers hate nothing more than the art of another weaver. none can be trained but you might be able to find a weaver to suit your needs. Of course, being ghosts, you can't really communicate with them. you'll need a catalog of weavers.
gotchas like to hid in dark places and terrify the unsuspecting. They know all your secrets.
ragwomen are roughly woman-shaped huddled creatures with large yellow teeth and grasping hands. They are incredibly quick and they love nothing more than to hold a baby and cuddle with children. Little grey, desiccated corpses hang from their belts on silver chains. they are very lonely and will follow anyone they think is interesting. They know a lot about children, and can tell you what some children in the real world are dreaming about right now. Generally not hostile, but if attacked, a ragwoman will defend herself, and her children will unwind their chains, clamber off her skirts and help her.
paper ghouls are white humanoid apparitions of almost two dimensions. They will drop down from a wind and slip into the chinks in your armor with ease, burrowing deep. Beware fluttering white clouds.
pink dobeys look like knobby pink clouds with enormous spindly legs and they eat and shit out one another. They are often used as means of relatively safe transport (rope ladders descend from rickety wooden carriages suspended from the dobey's knees, though ghost land wind patterns are hardly understood so the most reliable dobey-routes are educated guesses at best.
Some known ghosts
Most ghosts don't really have much of a form other than vaguely humanoid, with something a bit off. Many wear masks. Some ghosts can be communicated with, though even fewer of those are interested in talking about anything other than what they want to talk about. "Known" ghosts are ghosts of the singular variety, likely capable of communication and likely quite dangerous. There are thousands, these are just a few of them.
The hanged man
walks on his hands and will tell you one true thing about anything if you bring him a fully bound human being wrapped in a carpet or stuck in a bag. the living petrified are especially desired. lives in a tree that is always in bloom, in a place never touched by the tides or the change winds.
wrath looks roughly like one person curled in on itself. wrath is extremely difficult to find and is almost always hidden in a cupboard, wardrobe or the last place looked. scraping wrath's feet or elbows yields a spicy powder which, when snorted, renders the user a hulking, violent brute, of incredible strength and ferocity. Wrath addicts often spend their free time self-mutilating.
dressed like a police officer, teacher town guard or similar figure of authority whose neck terminates in thousands of waving frond-like antenna. Usually wears a burlap sack over its head with eyes and smiling mouth painted on, though the bag wriggles. Is always asking if it can come in. If you let it in, it will touch your face with its antenna; if it doesn't like you, you will be punished; it it likes you, it will invite you to a party.
the whispering crowd
a collection of dark, long, roughly humanoid shapes. They appear around someone fated to die and steal his dying breath. they know the last words of everyone that has ever died. it's unclear if they cause the deaths or they simply feed off the mechanic.
Mama Blue is dressed in a blue, floor length dress and will heal any wound, provide whatever help you need so long as you agree to be a good little boy or girl for her and do just as she says. she towers over you and even when she bends down close you cannot see her face.
Mama Green is long limbed, wild eyed and has long, snarled nails. She will offer help and will reward anyone who helps her enormously, but will often ask you to tell her how good she is, how pretty, how kind, how sweet. She knows when you lie. She doesn't take disappointment or liars well. she is taller than mama blue and her head is often in cloud cover so that when she bends down to speak to you, rain falls from her face. She walks among a field of silent, softly rolling envies that will part to make way for anyone looking for mama green.
Ghosts don't evict or subdue the will of those they possess. Instead, they simply play with them like a doll. The first sign your buddy has been possessed by a ghost is that he has cut out his tongue and snipped tendons and muscles at crucial points, making him a totally pliable witness for what's to come... (of course, because you can see ghosts in ghost land, all of this is totally obvious; in the real world, you likely can't see the ghost)
Ghosts are made out of ghost guts, a doughy material studded and woven with hair, wire, teeth, rotten food, glass, bones, rusted implements, old, ruined houses and anything else that's been discarded in the real world. Everything in ghost land is made out of this stuff except for water which is more or less the same. Ghost guts can be cultivated and turned into powerful drugs or spell components. Weapons made from petrified ghosts can cut shadows and whither limbs. Armor made from ghost guts will likely be highly immune to mundane damage, but will slowly seep into the wearer causing them to become progressively weaker, prone to obsessions, easily fooled and misled.
If ghost guts are used to supplement a will-altering spell, the spell has a 1 in 6 chance of being permanent (and a 1 in 6 chance of being horrible, causing madness, violence, turning the caster sensitive to ghosts [ie, easy to possess)).
Ghost candles are wax (or whatever) candles with a bit of a dead person inside, at the end you don't intend to light (can be any part). Light cast by a ghost light kindles a similar flame in any ghost that sees it (the ghost glows and a large flicker flames appears within it). A candle will captivate all ghosts that see it. They will be drawn by its light and will stare at its flame until it goes out. Can be extinguished as a normal candle, though there is a only a 1 in 6 chance a ghost will try to extinguish it. Burns downs as fast as a normal candle. Alternatives: scented ghost candles, ghost incense (doesn't illuminate a ghost), ghost plugnfresh things, ghost potpourri (doesn't illuminate them, ghosts will often play with potpourri and do weird shit with it (jam it into eye sockets, shove it into wounds; They like dead stuff).
prayer books closing your eyes, chanting a prayer and holding a prayer book aloft while you march through an area in ghost land will prevent any ghost from attacking you, though they may follow you and wait for you to stop chanting. they find you pretty annoying, at best.
pictures: Nakayama Masaaki from Fuan no Tane, dunno who did the second one, and Agostino Arrivabene (x2)