Practitioners of psuedo old-school summoning magick.

Theurgy is the product of the hermetic revivalist and spiritualist movements of the late 19th and early 20th Centuries, and a less than successful attempt to recreate the hermetic arts of antiquity and the Renaissance. Theurgists adhere to the ancient principle of “As Above, So Below”; and so believe that by summoning and communing the otherworldly entities that govern our fates they can harness their powers. Needless to say, it is impossible for anyone to contact the Cruel Ones or ascended archetypes, let alone control them, but Theurgists are very good at summoning demons. Admittedly, this is nothing all that out of the ordinary in the occult underground, but Theurgists also have a particular knack for getting hold of the exact demons that they want. This act of summoning signifies the Theurgists’ ability to command the forces of the unnatural, and it appears to be this symbolic demonstration of superior willpower that gives them the power to cast other spells: it’s all about attitude.
In theory Theurgists are extremely versatile, because they can basically create unnatural phenomena in pretty much the same way that demons can. Unfortunately, it is extremely hard for them to control the forces that they unleash, to the extent that their spells often do something completely unexpected, or just totally opposite to whatever it was that they had in mind. This may be because no two Theurgists can really agree about the true nature of immaterial spirits, their powers, or the confusing mish-mash of half-baked old school ideas that their magick is based on. This makes Theurgy extremely difficult and dangerous to use, and the few Theurgists who have mastered the school and lived have generally taken this to mean that they’re superior to just about every other adept out there. It is a sign of their elitist attitude that they also claim the right to call themselves hermeticists, although other adepts who are aware of this conceit generally shorten that to hermits.
Theurgy Blast Style: Bad things just tend to happen to people that Theurgists don’t like. They have bizarre accidents, they fall prey to strange illnesses, and so on. It’s difficult for Theurgists to specify a particular form of damage: they usually just have to cast the spell and let fate do the rest.

Stats.
Generate a minor charge: Conduct a ritual which summons the departed spirit of a specfic person and then control it. Something intimately connected to the dead person in question must be included in the ritual, such as a personal secret or something that belonged to them, or else all you get is a random demon, as per the summoning rules in Unknown Armies, p147-148. Once you’ve contacted a demon there’s nothing to stop you summoning and controlling the same one more than once, but there’s a distinct risk attatched to this. If you screw up on a subsequent summoning, then the chances are that you’ve got a demon inside your head that wants to get its own back for being bossed around before. You can be safely assured that this will be no fun for you at all. To limit such risks, every Theurgist is taught a minor ritual which allows them to summon and control demons, and many of them also have working knowledge of Authentic Thaumaturgy to facilitate this.
Generate a significant charge: Summon and control the spirit of someone specific and important enough to have been a minor public figure in at least one country. Alternatively, summon the departed spirit of an avatar or an adept who died while holding a significant charge. The same minor ritual can be used for this, and once again something intimately connected to the spirit is needed to achieve the desired effect.
Generate a major charge: Summon and control the spirit of a major historical figure, a godwalker or an adept who had a major charge when they died. For really famous people, you need to have some really serious leverage, such as conclusive proof of something that will irrevocably change the way the world remembers them. Given that godwalkers and major adepts aren’t always historically famous, no one is exactly sure how to go about finding the information needed to summon one of them.
Taboo: As above, so below. It is a sign of weakness and subservience for Theurgists to serve any mortal except for their mentors in the school, and so you lose all their charges if you ever become contractually obliged to work for an employer in a paid job, either full-time or part-time. What’s more, you can’t gain any more charges for as long as you remain in employment. It’s perfectly acceptable to be self-employed, but it’s no coincidence that many Theurgists are independently wealthy, and accustomed to using magick to supplement their incomes.
Random Magick Domain: There are lots of vague assumptions and no definite conclusions about what Theurgy can do. It’s generally agreed that Theurgists can do just about anything that demons can, and some are of the opinion that you need demons of a particular sort to get a particular sort of effect. For example, you might need to call up Jack the Ripper or Ted Bundy if you want to kill someone. Interestingly, while Theurgists can heal or harm bodies, they have little power over hearts and minds. This magick can’t be used to control people, influence them or gain insights into their thinking: Theurgists must rely on natural instincts for things like that.
Starting Charges: Theurgists start off with three minor charges and a minor ritual for summoning and controlling demons.

Theurgy Minor Effects.
Theurgy tends to be such a personal form of magick that no two adepts are likely to be able to use the same formula spell. You can work out their own formula spells over time, of course (see the rules for this in Postmodern Magick, p. 40-41), but you’ll be using something a lot like random magick until then. Basically, for 1 minor charge you will be able to cause one minor unnatural phenomena to occur somewhere within your line of sight. The GM decides what happens next (see Unknown Armies, p.144-145 for some ideas), and the results are pretty much beyond your control unless you want to take any of the following negative shifts to your magick roll:

Desired Effect Shift
targets a specific location -10%
targets a specific object -10%
is harmful to an object -10%
is beneficial to an object -15%
reveals information about a location or object -15%
will indirectly harm a person (minor blast) -20%
is precisely specified -10%

These penalties are cumulative, and if the GM is feeling malicious or just disinclined to allow PC Theurgists each one adds an extra charge to the effect. Alternatively, if your GM wants to do it this way instead, paying 1 extra minor charge will allow you to avoid one penalty, and it may also be possible (once again at the GM’s discretion) to avoid the penalties altogether by using 1 significant charge to cast the spell instead. Whatever you decide to do, something will still happen if your modified roll fails but would have succeeded without the penalties. In other words, something will happen if you still manage to roll below your unmodified Theurgy skill. But it may not be what you were hoping for.

Theurgy Significant Effects.
For 1 significant charge, you can trigger a random significant unnatural phenomenon, as described in Unknown Armies, p.144-146. As with minor effects, you must either take penalties to your roll, spend more charges, or both, in order to get specific results, otherwise you will cause random weird stuff to happen to random people. These effects target living beings much more than the minor ones do, and it is supposedly possible to travel astrally and possess other people with significant charge Theurgy magick. Unfortunately, there are no formula spells for pulling off stunts like these; if you want one, you have to invent it yourself. For significant effects, the following cumulative modifiers apply unless the GM allows you to spend extra charges to avoid them:

Desired Effect Shift
targets the caster -5%
targets an unspecified person in a specific location -10%
targets a specific person -10%
targets several people -10% per individual
reveals information about a person -15%
is beneficial -20%
is directly harmful (significant blast) -15%
is precisely specified -15%

These penalties can change at the GM’s discretion, as significant effects are even less predictable than minor ones. As with the minor effects, though, you still cause something to happen if you fail a magick roll that would have succeeded without penalties. If you blew lots of charges trying to get a specific effect, the results may be pretty catastrophic.

Theurgy Major Effects.
Anything goes. If it goes wrong, pray.