Hm, hosting.
Let's see, the whole idea is that spirits are creatures of chaos, while humans are creatures of order. Human physiology works by keeping a wide and varied set of physical and chemical parameters at a constant value (what value this is can vary). Spirits have no recognizable physiology, and their forms are highly malleable, and they don't even have a metabolism, let alone homeostasis.
So, when a spirit moves into a human, the chaos of the spirit wreaks havoc on a human's physiology, scrambling all those carefully balanced systems as it alters the human's vital parameters and tissues wildly in order to optimize for whatever task is required. Usually kicking ass. It's not always deliberate, either - the spirit jumping into the human causes their systems to seek equilibrium similarly to the pathway causing universal properties to mix between the spirit plane and the material plane, but much faster.
When the hosting ends, it is up to the spirit to restore the human to a viable state - the longer the scrambling has occurred, the more knowledge of the host and skill of the spirit this requires. If the reconstitution of the human fails, the human experiences some or all of a list of varied symptoms that end, naturally, in death, or perhaps something even worse. By allowing the hosting, the human trusts the spirit (who, it should be said, is likely to be many things, but is definitely not usually all that familiar with human anatomy and physiology) to put them back exactly the way they're supposed to be. The first few tries at this must necessarily be with a short hosting time, as otherwise the spirit will very easily lose track of the changes, and the reconstitution invariably will be incomplete, giving the human a series of unusual features that may, if the human helps, be corrected at a later time. A side benefit once the human and the spirit are well-acquainted could be spirit-directed healing of the human's body. And finally, once the spirit has learned fully the body of their host, you could even have them grant the ability to alter the human's body beyond the template memorized by the spirit - like shapeshifting, for instance, or permanent increases (or decreases) to physical or mental (though mental would probably be a much riskier proposition) abilities.
You can even let it work the other way, where the human can, through certain thought patterns, such as elaborate consideration of their inner workings, or recognition of the physical impossibility of the spirit, accidentally or deliberately impose structure on it, altering its being to a great degree, and even diminishing their powers and robbing them of malleability if they desire it - the spirit cannot survive such a thing, as by being robbed of its malleability entirely it gets placed squarely in the realm of their physiological plausibility being a real, physical question, one that usually has a very emphatic "implausible" as the answer. By entering a human being, a spirit trusts them to let it do its work, as by trying to force their will on the spirit, they can kill both themselves and it. And if a spirit forgets itself inside a human being, the changes can sneak up on them anyway, as per the attainment of unnatural equilibrium - the first psychodynamic rule, if you will.
EDIT: an important possibility - the familiarity of a spirit with the human body (and the specific human's body) decides the normal maximum mostly safe time for hosting, while the threshold of change that renders the spirit non-viable (up until that point, the spirit can change back into its preferred form more or less quickly) decides the theoretically possible maximum safe time for hosting.
A spirit can force itself on a human being - these are usually foolhardy, ignorant or downright murderous spirits (or a combination of the three), and their optimal target is an unaware human being, who they can mold to their will while risking little repercussions. A learned human being, however, is always a danger to a spirit, especially an unsuspecting one, as they can easily force the spirit to suffer much more damage immediately than the spirit can do while familiarizing itself with the new body and how to influence it. Granted, it's not likely to end well for either of them anyway. But it does render most spirits, who do not know such a thing as death at all in their home realm, rather paranoid of trying their abilities on a human they do not know.
EDIT: And if a human is partnered with a spirit, then the change that a random attacking spirit brings can be reversed by their spirit partner if needed, so being partnered with a spirit affords some measure of safety from other spirits. And should a human wish, they can even do a reverse hosting - jump into a spirit's space and start offensively twisting them into non-viability. This is a desperate tactic usually, however, since it's very likely to kill the human attempting it.
Now, some humans, once told of what a spirit does to the body if not bearing in mind all of the above, may recall odd cases of people turning up unrecognizably mutilated under mysterious circumstances every now and then (probably in an area near the pathway, where spirit density is highest), and instantly be rather leery of letting a spirit mess with their bodies. It is up to the spirit to convince the human of its benevolence and for the human to take the plunge and risk their life for the power that spirits can grant.
As for what a spirit chooses to be like in their own world (near the pathway where such things have meaning) is usually inspired from the mortal world, whether it be a form that waves like a flag and shines like a neon sign, or a real-world concept like Demogorgon, Lord of All That Swims in Darkness, or even a lowly demonic imp in an imagined hierarchy (perhaps forced or even willingly playing into such a role at the behest of their fellow spirits). One would think that this roleplayed mirror of reality would not hold that much sway over a spirit's psyche, but if conversations reveal anything, it is that they suspend disbelief to a degree unheard of by humanity - their token measure of structure on the other side is literally the only thing they can hold on to there that anchors them to any sort of solid, lasting shape, a beacon that their chaotic mind uses to orient itself and form semi-coherent goals.
As for grossness, I don't suppose a human being would be much more awful to a spirit than, say, a potato would be to us, except with the respective bodily complexities reversed. We find the human body gross because it reminds us of our own culturally improper physiological functions, and its structures unnerving because to see them indicates that someone was gruesomely harmed to bring you the sight of them, reminding us of our own mortality. A spirit would have no such established reservations, I would guess. Perhaps they would be confused, being unfamiliar with it all. Or maybe they'd even find it cool. Do you find transpiration or guttation gross, for instance? Or absorption of water through roots? The process of shedding leaves? Pollination? Etiolation? It's only when you start to draw parallels to our own physiology that they can be understood as properly gross, which the spirits can't really do, what with having no physiology.
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Topic: Lore thread - Of Substance and Spirit - Think we're ready to make characters? (Read 17876 times)