* Magic in these worlds, including our Earth, once that starts happening, is fairly similar to the Path/Book system presented in GURPS Thaumatology and the narrower Ritual Path system from GURPS Monster Hunters (although priestly folk on Fantasy Earth might use the 'prayers and miracles' system presented in GURPS Powers: Divine Favor, if their gods are powerful enough), or in some cases, a version of the Symbol Magic systems from GURPS Magic and again, GURPS Thaumatology. The powers behind that magic, however, whether divine or arcane, are psi powers, and these define the limits of what can be done - though if you're creative, you can still do a hell of a lot. Without going into multiple pages of detail, very few magic-users can cast a spell by just muttering and gesturing for a few seconds, or pointing and saying the name of the spell in some obscure language - and most of those who can, are only able to do it for a few spells they know very well. Instead, they spend a few minutes to a few hours (depending on the skill of the caster, the complexity of the spell, various environmental conditions, and whether they're summoning an existing spirit, or generating a new one) chanting, gesturing, and possibly singing or dancing, in a ritual circle or similar space prepared with various symbols, and then either the magic starts to happen, or the mage take up a charm that the ritual has loaded the magic into - these temporary magic items last for days to weeks, and are generally broken when activated.
Generally, the magic is Effect Shaping, but some specific styles may use Energy Accumulating, instead. In the latter case, the 'energy' is lots of minor thoughtforms.
The act of casting a spell most often involves creating or summoning a thoughtform (see link in the tl;dr section below) - a sort of psychic computer program running on the medium of the collective unconscious (the astral plane, if you ask most mystics). Generating a new thoughtform tends to take longer than summoning one, unless you need to negotiate with the one you summoned (OK, technically, you're generating a thoughtform even when you summon another, but all that one has to do is get a spirit's attention, which is why it only takes a few minutes or less to make). Most newly created thoughtforms, especially those made for specific, limited tasks, tend to be very lacking in willpower, and in many cases, fade from existence fairly quickly (those created to get another spirit's attention tend to be absorbed by that spirit, increasing its power by a miniscule amount, which encourages it to at least pop by and talk). 'Free' thoughtforms, those that did not fade away for various reasons, are spirits, and the most powerful spirits are called gods. Most of them were generated unconsciously by groups of people, but a few were created by sorcerers, and grew beyond their origins. Permanent or multi-use magic items are fetishes (no, not the sexual kind): objects which have a thoughtform bound to them, whether formed by the caster, or summoned and trapped, willingly or otherwise. Spontaneously created spells are possible with the right stimulation (such as Haversham's Enhancer, or the comet that came to Clockpunk Earth), but tend to be very limited, quirky, or both. Intentionally designing spells is much like writing a computer program, in a very eccentric language, in your head. Meditation is often involved. Thoughtforms are capable of transitioning the 'quasi-wormholes' connecting the various Earths, but their arrivals produce only a very weak particle spray, difficult to detect with 2012 instruments - unless a whole bunch arrive at once, of course.
Thoughtforms sustain themselves, at least in part, by absorbing psychic energy directed at them, usually focused through various emotions. By itself, this doesn't harm the person being fed on - you're already emitting the energy, whether they eat it or not. Different thoughtforms often prefer different 'flavors' of emotion, and this affects their appearance and behavior - a spirit that feeds on lust tends to be attractive in some way, and acts friendly and/or seductive, those who feed on fear tend to look frightening, and act creepy or hostile to various degrees, and so forth. This is also how spirits can be harmed: those too strongly adapted to one or two emotions are often 'poisoned' by others, and nearly all spirits can be 'injured' by the desire to do harm to that spirit, especially when directed by a reasonably strong will, at mêlée ranges (not that where you see/sense the spirit is it's true location - it's in the collective unconscious, which means it's in your head, just like it's in everyone elses' - but disembodied spirits are much harder to hurt, if they don't notice you're trying to hurt them, and spirits generally pay the most attention to the location they appear to be in, whether in the real world, a dream, the astral plane, or even in a virtual world on the computer, for those that can get there). The easiest way to focus such an attack is to attack the spirit's apparent 'body' with a mêlée weapon or unarmed attack, while strongly willing it to 'Be Injured' or 'Die'. Mêlée weapons are useful for this purpose because the wielder's spirit (ki, prana, aura, whatever) tends to extend over the weapon, when one is sufficiently skilled with it. This doesn't really apply to ranged weapons, or rather, it doesn't apply well to projectiles, once they're no-longer in contact with the user (though you can extend the range a bit by attaching a wire, thread, or similar, if you practice enough with the modified projectile). Of course, you can turn a projectile into a charm or fetish with the property of 'harms spirits', but this works best with larger objects like arrows or throwing knives - it's hard to convince a powerful damage spirit to cram itself into something as puny as a bullet (no matter how fast it goes), especially as damage (and resistance to it) is dependent on the relative willpowers of the inflicting and receiving spirits. It's much easier to harm spirits which are 'permanently' anchored, willingly or not, to physical objects (fetishes, in other words, but not charms), as they will generally be injured by damage to their host items, and may be killed by the item's destruction, or banished to the depths of the collective unconscious (the Inner Astral Plane, as mystics call it), for some time (depends on lots of factors, but is generally measured in years, decades, or centuries, due to the trauma of being 'killed').
Some spirits who prefer specific emotions will merge, temporarily or permanently, with thoughtform-copies of some fictional or real being, in order to better evoke their preferred emotion. The spirit thus becomes, for example, 'Bugs Bunny, if he were a laughter spirit' (not much change, there), or 'a dementor, if it were a fear and/or misery spirit' (which again, isn't much of a change - in fact, it makes more sense in this context than dementors eating happiness), or 'a clumsy little puppy, if it were a d'aawww(g) spirit' (no, I'm not going to give you examples of who/what a lust spirit might merge with - you've already got a dozen ideas in your head, you don't need any help from me). Some of the friendlier spirits will also merge with minor elementals or other nature spirits, so that some specific person they 'like' will find them more useful, and wish to keep them around. Yes, for some types of spirits and 'donors', this can get various types and levels of creepy (and not just with lust spirits, either) - they're not human, and even when merged with a ghost or other fairly human-like thoughtform, don't
quite think like humans do. The humans that contract with them don't usually have that excuse, but with many spirits, it's better than letting them run wild.
The spirits' ability to effect the physical world requires them to convert physical energies for their own use, and has nothing to do with how they sustain themselves as spirits. The most common energy source seems to be heat, which explains why magic items and materials are often cool to the touch, and haunted houses frequently have unexplained cold spots - the fact that this appears to violate thermodynamics doesn't bother the spirits, at all. The strongest energy source is quite unknown, although it might be a form of what OTL scientists currently call '
dark energy'. This is the apparent source of the extra energy from Clp-Earth's springmetal, and the enhanced fuels on Dp-Earth and Stp-Earth.
Continued in next post. Sorry this is taking so long.