Magic

Summary
(from Game World Index)

Theurgy is a mystery. According to the Ecclesiasts, it is a gift of power from the Blessed Angels to especially worthy individuals who can be trusted to defend Xthonos's sacred Order. For anyone not chosen by the Angels, attempting Theurgy is a grave sin. There's an old-fashioned word, "goete," for a black magician who consorts with Xaos-demons to duplicate the results of Theurgy; "the Abomination of Goety" is one of the worst crimes listed in the Karagond Canon.

The one thing everyone knows about Theurgy is that it requires blood. Harrowers yield rarefied "aetherial" blood, which is particularly potent. The reasons for its potency are rumored to be both sacrificial (drawing power from the anguish of the victims) and alchemical (involving a closely guarded refinement process). Theurges generally keep bandoliers equipped with dozens of small phials, which they pour or break into their hands to fuel their powers. As long as their blood supply holds out, Theurges can move people or objects, heal illness, fly, turn people to dust, conjure immense fireballs out of oil of vitriol, and a wide range of other marvels.

Most of the Theurgy you've heard of is fleeting, quickly consuming the blood that fuels it. Some spells have permanent effects; the Hegemony's Plektoi shock troops are Theurgically altered giants with leathery skin and stone-hard bones. But making Changes of that sort is supposed to require much more blood. It's almost impossible to imagine the quantities required to maintain a massive enchantment like the Border Wards or the Floating Palace of the Thaumatarch. You wouldn't have thought there was that much blood in the entire population of the Hegemony.

How People Think Theurgy Works
The ritual of sacrifice (including the death of the person concerned) is supposedly essential to turning normal blood into aetherial blood. That’s part of what makes people so afraid of Goetes… if they aren’t using Harrowers, what kind of obscene sacrifices must they carrying out to get their power? Since the refinement process is a sacred secret, though, you have no idea of why this would be the case. And if you were caught asking the question, you’d be punished for sacrilege and/or sedition. (source)

How Theurgy Works
"When I look at a blade, I'm not just seeing what it's made of.  It's like I'm directly seeing what it was made for—cutting, severing, injury.  All together, in a sort of blur.  And most of what's in the blur, I don't recognize."

In Game One (Uprising) it is possible to learn/deduce the following about Theurgy:
 * When someone enters a Theurgic trance (For the MC, through the use of the philosophical litany), and then use either rarefied aetherial blood, or one's own living blood, perception and senses are altered.
 * In a Theurgic trance, you can perceive the telos, or purpose of a person or object. Teloi refers to the heart of something's nature, the goal towards which it moves, that which fulfills its reason for being, such as the striving of elements to rise (air, fire) or fall (earth, water), or the spear's purpose of piercing.
 * Philosophical texts use the word 'unchanging' to describe teloi.  To alter anything's 'final cause' would be to destroy or unmake it. A practitioner of Theurgy would be capable of making Change for anything, as long as they are capable of understanding what they see. (Otherwise it remains a blur, too complex to grasp.)
 * It seems to be easiest when you try to make a Change that goes with the grain of a thing's nature, rather than outright reversing its telos, as implied by Horion Leilatou's observation, "...encouraging the elements of air and fire to rise would be easier than levitating an object made almost entirely earth or water..." An example is speeding up the ripening of crops rather than withering them.

Much more will be learned in Stormwright Chapter One. For now, however, those facts remain spoilers.

Flying Couriers
Theurges are effective at travelling long distances very quickly, thus making it ideal to have them assigned as messengers and couriers, who send relay messages between different places. The Phalanghites utilize them to send reports to superior officers from far distances during a battle or when putting down insurgencies. They are also capable of transporting people themselves on occasion, notably Archon Phyrgia Leilatou.

Military
Theurges can make use of simple telekinesis to throw enemies off their feet or simply off balance, which is particularly dangerous in mountainous terrain. Theurges also make use of vitriolic oil to conjure up fireballs to engulf enemy forces. On the Hallasurq border, Theurges can sharpen Phalanghite blades in order to cut through anything that stands in their way. Plektoi handlers are often sent in at crucial moments to severely damage enemy morale and disorganize them.

Techne
Due to the convenience of Theurgy, techne can be utilized in order to accomplish large-scale industrial initiatives, such as Harrowers. Due to Theurgy, alternate sources of powering techne have been disregarded in favor of using blood magic instead.