GURPS Discworld presents an essentially simple approach to running Discworld magic in games. There is good reason for this; Discworld games should be funny and fast-moving, and any complex treatment of game-mechanical matters is likely to get in the way.
However, the fact is that Discworld magic is a complex affair, with intricate (if often obscure) rules of its own, and standard GURPS magic, while close enough for most people, doesn’t quite fit the picture. On the other hand, there is a system of magic rules available for GURPS that does fit a little better, at the cost of a little complexity. This is the system presented in GURPS Mage: the Ascension. This note explains how to use this system on the Discworld, for gamers who’d rather do it right than do it fast.
All characters who intend to use magic (don’t bother with the "k" -- most Discworlders have enough trouble with five-letter metaphysics, although some dubious foreigners and self-appointed witches might insist on the distinction) must have one or more levels of Arete (rename it Wizardry if you want to keep the Discworld flavour). Almost all such characters also have Awareness, which wizards call "Octarine Vision" (the ability to see the "Colour of Magic"), although some half-baked types get by without. Competent spell-casters also have a few levels of Avatar, although most of them don’t bother their heads with abstract questions of reincarnation, and just regard this as a mechanical matter of how much magical oomph they can hold in reserve; treat it as something that can be bought freely with experience. (Don’t bother with Epiphanies or Seekings.)
Some witches and shady foreigners have a couple of levels of Arcane; most wizards would regard it as a Disadvantage, as they prefer to be noticed and admired, but some theoretical quantum metaphysicists acquire it, as their insistence on talking shop over dinner leads other people to try to forget their existence.
Nodes are rare, costing double the listed points cost, and have several built-in disadvantages. For one thing, other magical characters will be fascinated by them, and most unhappy if you don’t want to share. Worse, however, is that any point where reality wears thin in the Discworld universe is likely to attract the attention of the Things From The Dungeon Dimensions -- and they may well be able to get through there. Incidentally, Meditation skill, which is pretty much a necessity to really exploit a Node, is mostly unknown except among people who are far too enlightened to actually use it for anything useful.
Don’t bother with Immunity to the Delirium; werewolves are perfectly well understood on the Discworld. Destiny, Dream, Enhanced Time Sense, and Immunity to Paradox are all rare or unknown; a few characters may have discernible Destinies, but anyone who can sense this will usually react badly to it, as most of them will be jealous, and most other Discworld inhabitants are painfully aware how dangerous it is to be in the presence of greatness. (Anyway, it shouldn’t be limited to the magically adept; characters like Carrot have far more blatant Destinies than, say, Rincewind.) Hobgoblins don’t appear to be a common problem for Discworld magic-wielders, but they actually fit the nature of the setting extremely well, and so should probably be permitted as an optional Disadvantage.
Use the standard GURPS Mage spheres, but try and pick ones that fit the character concept and background. Some of them may work a little better than others; for example, Discworld magic is notoriously bad at creating permanent physical objects, so high levels in Matter should be rare and probably hard to attain.
Wizards, being mostly concerned with power and showy stuff like teleportation, tend to be big on Correspondence, Entropy, and Forces, sometimes backed up with a little bit of Matter. Witches, on the other hand, are subtle enough to know how much can be achieved with Life and Mind, plus some slightly more subtle applications of Entropy. Both groups usually pick up a level or two of Prime, to fuel the other stuff, but the tricky and variable nature of Discworld metaphysics makes higher levels of this sphere notoriously hard to attain; wizards who try invariably end up talking about quantums, while witches just disappear into their cottages for months at a time and lose the essential respect of their neighbours. Time is a little-studied sphere, although a few showy types may find it of interest.
That just leaves Spirit, which is the least trusted field of study in Discworld metaphysics. Frankly, few wizards like the idea of dealing with things that are better at juggling the supernatural than they are themselves, and witches mostly have more sense; a botched exercise is too likely to conjure up something like a True Dragon, High Elves, or the Things From The Dungeon Dimensions. The only group in the civilised world willing to take this sort of risk on a regular basis are Demonologists, who even wizards regard as basically daft.
Except that, well, many witches and high-level wizards permit themselves the odd minor summoning, purely for reference purposes...
In the uncivilised world, shamans are much given to a low-key, reasonably safe style of Spirit magic, but then they are also much given to altering their consciousness with local fungi, so they are quite used to talking to beings who not only aren’t there, but really couldn’t be anywhere. The other class of people who always have a level or two in Spirit are mediums, but these don’t really count as spell-casters, being untrained "natural talents" (when they aren’t outright frauds), and usually embarrassingly lower-middle-class in their view of life. (The best of them may also have enough control over Time to function as precognitives.) As all this should indicate, Spirit magic is usually used in relation to the caster’s favourite sort of supernatural being, as summoning something you don’t know much about is just too obviously dangerous.
Minor magic-workers around the Disc may learn or pick up almost any Sphere or combination of Spheres, but usually have a coherent theme to their magic. (For example, the tiresomely practical wizards of Krull tend to concentrate on Forces and Matter.)
Like the Traditions defined in GURPS Mage, the different styles of Discworld magic-worker handle their magic very differently. In fact, the differences between them are so deep as to make some people think that they do different sorts of magic.
To begin with, however, it should be noted that the Discworld is mostly more tolerant of magic than our reality. The vast majority of Discworld inhabitants believe in this stuff completely, and indeed rather expect it. Thus, all castings are usually treated as Coincidental for purposes of determining Paradox, although only truly Coincidental magic receives a skill bonus for Arete.
Except... Discworld folk are terribly conservative in many ways, and tend to associate magic with certain accoutrements. Specifically, they mostly expect pointy hats, plus robes and staves for wizards, or warts and attitude problems from witches. It is possible that someone who had the temerity to dress and act like a regular human being while working magic would actually confuse on-lookers, and maybe even reality. GMs should use at least the threat of vulgar-magic-level Paradox effects to deter such inappropriate behaviour.
This may also help explain Discworld magic-wielders’ many other quirks. These include a fondness for foci, even among witches who really don’t seem to need them, and a positively fetishistic approach to rotes (always referred to as spells) among wizards.
Almost all wizards use spells, almost all the time. They have books full of the things, and talk in dense jargon at the drop of a pointy hat. If this is really largely unnecessary -- well, don’t try telling them. To be fair, the ability to increase one’s effective skill level with a spell is quite handy. Occasional incidents of wizards using what look like coincidental effects have been reported, but this may well be limited to those of a high enough level that they are used to doing as they please. It is also possible that Discworld magic can only be used coincidentally by characters who have surpassed the need for foci.
Witches are much more given to fully coincidental effects; they do also use spells, but even these tend to be relatively low-key. It may help that their foci are sometimes less obvious to onlookers. Headology, in this system, is coincidental use of Mind effects.
Discworld magic-wielders use a huge range of foci, without any absolute standardisation, but certain items are at least well known.
First on the list is the wizard’s staff, which can be regarded as a focus for every sphere, except Spirit. For that, wizards and demonologists use complex magic circles and Octograms, complete with candles and suchlike. Other complex spells have their own specific focus requirements.
Mirrors are useful for Correspondence magic, especially passive viewing effects; using two mirrors is notoriously dangerous, although it can add power to effects from many spheres. Witches and voodoo-workers also use dolls and dummies for attacks with a Correspondence-based aspect (in the old sense of "stick a pin in the doll, and the person goes ouch").
For Spirit effects, witches use moderately complex paraphernalia; cauldrons, wands, candles, and so on. However, they can get away with some very crude versions of the appropriate materials -- see Wyrd Sisters for a demonstration.
And incidentally, a witch’s broomstick is, of course, a Talisman with the Forces-4, Prime-2 effect of flight, and dwarfs can somehow create such devices -- presumably because they are basically "coincidental" -- without having appropriate sphere ratings of their own.
The place where Discworld magic diverges furthest from the GURPS Mage standard is in the matter of Paradox, if only because it seems to be less of a problem for them. But that is "seems to be," not "is." At lower levels, and most of the time, witches and wizards can do what they like; but if they over-stress the structure of reality, they suffer the attentions of the Things from the Dungeon Dimensions, which are not exactly paradox spirits -- just very bad news (see GURPS Discworld). More subtle, but equally startling effects include permanent transformations of people who get in the way, as in the case of the Librarian. GMs should set up all paradox effects as seems appropriate for the plot and proportionate to the magic used, rather than attempting hard-and-fast rules.
Borrowing: This common witch technique is, of course, a Mind-3 effect.
Headology: This is coincidental use of the Mind sphere at levels ranging from 1 to 4.
The Rite of AshkEnte: By a curious quirk of Discworld metaphysics, summoning Death is only a Spirit 1, Entropy 1 effect, but it always requires at least a couple of people and a focus consisting of several elements. Death can and does turn up anywhere, with or without invitation, but if he’s going to be invited, he expects certain minimum formalities.
Zombie Creation: This is a complex Spirit-4, Prime-3 effect that recalls a spirit from the afterlife, or stops it going there, and binds it back into a magically-empowered body. Matter-3 may also be needed to restructure or re-make the body, and Mind-2 or Mind-4 may be used to manipulate the zombie’s behaviour to varying extents.
In "Discworld/Mage" magic, a Sourcerer is a character with a lot of points and damn few limitations. Okay, that’s obvious, but the details merit some attention. Being able to tap directly into the raw magic of the universe, the Sourcerer can achieve any level you find appropriate in any Sphere, including Prime, and doesn’t need any Foci. Once he’s launched on his career, he soon gains an image and self-image as someone who can do anything, so all effects that he creates are fully coincidental. Of course, a Sourcerer is above such wizardly trivia as spells (rotes), but that’s hardly a problem. And by advancing to the highest levels in Prime, the Sourcerer can become a sort of "walking node", which is convenient for any wizards in the vicinity. However, little of this necessarily helps against the Things From The Dungeon Dimensions, who are likely to notice the gash in reality fairly quickly...
In other words, don’t include a Sourcerer in your game unless you want to end it soon, or unless you have a very effective get-out available. PC Sourcerers are right out.