Ghosts & Ghost Damage

This, but like, more see-through

Common Knowledge

Ghosts, and spirits in general, are the souls of the dead. They, for the most part, cannot effect the world and, in turn, cannot be effected by the world. They are frightening, but not in the way that, say, a tiger is frightening. They are frightening, perhaps, because they are judging you. To see, or, more accurately, feel their presence is to know that someone is watching what you are doing, and may pass that information along. That's how it generally starts at least. 

How Ghosts Affect You

The reason that ghosts sometimes start by inflicting a feeling of being watched is to draw their target's attention to the unseen world. The more you are focused on the unseen world, the more that ghosts can affect you, for they are its denizens. This isn't exactly a cognitohazardous effect. It's actually less like becoming aware and more like being drawn in, and they have many ways of doing that. Most ghostly attacks draw you further into the unseen world in addition to any other effects.

Ghosts can also attack your souls directly, in a similar manner to Death. This makes you easier to fight, both in the visible and unseen world.

How this Works

1. Ghost attacks. They deal an amount of normal damage (maybe a d6 or something) that is capped based on the amount of Unseen damage that character has (they can only do unseen damage+1 max). Then, regardless of the cap, they might do some amount of Unseen damage (which acts like another hp or sanity bar or something).
2. Then, the player rolls a d20. If they roll under their Unseen damage value, they make a death save. If they fail that death save, mark it on their character sheet. The more death saves marked, the less visible the material world becomes to them. 1 and the world becomes lightly obscured; 2 and it is heavily obscured; 3 and the character becomes blind, or maybe unconscious. When they fall unconscious, their death saves are removed at DM's discretion. 
3. If the character is still conscious after this, the unseen damage makes it easier for them to see the unseen world. 5 damage and spirits are only lightly obscured instead of heavily. 10 and spirits are completely visible. 

Spirits can also do unseen damage by their mere presence. They can choose to deal 1 damage per 10 minutes, but this damage does not provoke the check. Cap that out at ~5 damage total. Tell the players whenever they take damage this way, and they'll probably roleplay that fear of the unseen pretty well.

Side note: Death Combat (see link above) does not occur when fighting a ghost (unless you want it to). 

Example

Frodo was pursued by ring-wraiths for a long time, which drew him into the unseen world badly enough that they were able to stab him, which both brought him further into the unseen world and severed one of his souls. He had ~10 Unseen Damage and had suffered 2 failed death saves (I.E. he could see the unseen world clearly and could hardly see the material world) by the time he was brought to Elrond, who gave him a soul made from the spell [light]. The morgul-blade gave him some permanent unseen damage, though, which only grew over time.

Why Ghosts are Like This

Humans have seven souls (maximum, some have less). They are: mineral, vegetable, animal, memory, personality, reason, and divine.  Three of these souls are entirely in the material world. Depending on who you ask, one or two of these are entirely in the unseen world. This makes people somewhere around one seventh in the unseen world already. This is why ghosts are able to affect them at all, and why people can dimly see/sense ghosts. 

Note, however, that these souls are just generally found in the seen/unseen world. Certain rituals can make more physical the memory, personality, etc.. In the same way, ghosts make less physical the various souls (except the mineral, which is literally all physical with no spirit bits. same with making physical the divine soul). Like death, they work their way down. Depending on who you ask, they work first on either the reason or the personality, which is why people act weird around ghosts (some people say that it depends on the person, because some people have very lofty goals which make them already a part of the unseen world. This is why some good people can act afraid but still be courageous).

While humans have 6-7 souls, ghosts can only have 4 max. The souls below the divine soul but above the physical soul can't (normally) exist on their own. Thus, most ghosts have a divine soul at least, but other than that can have any non-physical soul regardless of order, giving rise to non-person memory ghosts, classic amnesiac ghosts, and others. Ghosts that have animal souls don't generally have any other souls, and they are called Specters (though this can be used as a general word). Most people don't have enough cohesion to form their own ghost after death, but if there is a great enough number of other free floating souls (I.E. lots of death) around, they can form a collective ghost which can almost function. These collective ghosts might have every soul above vegetable at the same time, but have a difficult and tortured existence due to their terrible death and the predations of lesser ghosts who long to be whole.

Physical forms can be made for spirits who have proved themselves to their goddess either in death or in life. This is where Angels, Demons, Powers, Archons, and Elementals come from. Most ghosts don't achieve this, however, and the whole Great Sea has only 99 Archons and 3 Powers. You can also force a ghost into a physical form with a ritual in so they may be more easily defeated.

When killed, the souls of a ghost are scattered like any other being. This generally results in reincarnation

Happy Spooktober