Theurgy (spoilers)

When you meet the renegade Theurge Cerlota de Viore in the Xaos-Lands in Game Two Chapter One, she will explain in more detail how Theurgy actually works.

Aether
Theurgy is fueled by aether—the fifth and most sublime element, the substance of which the stars are made. The presence of aether can render teloi malleable (by analogy to how fire, properly deployed, can render otherwise solid material substances malleable).

If you asked most people in the Hegemony, "what does the 'aetherial' in aetherial blood mean?" they'd say it's basically a synonym for Angelic. It doesn't have any particular meaning other than "holy and weird like the Angels."

Only a few are aware that it's not originally a religious term, but alchemical vocabulary that crept partway into popular usage (without laypeople ever quite understanding what they were talking about)...before it became suddenly clear to the relevant alchemical elite that they needed to make sure that no one outside a very limited circle of practitioners ever found out what "aetherial" really meant. It was too late to keep people from saying it, but not too late to mystify the term beyond recognition.

As that small cadre of early alchemists discovered when they began successfully probing the reasons Theurgy worked, human blood has a relatively low aether concentration, which it loses almost instantly by sublimation once the blood leaves the body. A living person using their own living blood makes fairly effective use of its teleomorphic potential. Early alchemists tried and failed to preserve that potency through preventing sublimation. None were able to come up with anything that fixed a meaningful volume of aether, not enough for more than a party-trick level of Theurgy--nothing that could really compete with a dedicated Theurge using their own blood.

Thaumatarch Hera was thought to have been the one who finally cracked that problem, through a carefully guarded refinement process. In fact, while she did discover a new alchemical pathway for reducing sublimation, her truly crucial insights were (1) that other human bodily tissues have a much higher aether concentration than blood (If you go kamikaze and pull down a mountain in Game 1, you're basically drawing on a whole bunch of that aether, not just the stuff in your bloodstream) and that (2) it would be possible to mask a fairly large-scale process of harvesting those tissues thanks to the Krypteia traditions of helot-hunting in certain city-states.

Thus began the conquest, and the Thaumatarchy.

Those aether-dense tissues – the brain, mostly, but the eyes and bone marrow also have enough to be worth harvesting – lose less to sublimation before the aether can be "fixed" through an alchemical process. (That process begins in the Harrower, yielding an inert precursor that can later be refined into usable form). Blood remains a useful natural solvent, contributing a bit to the solution's potency... and "blood" makes an even more useful, politically tolerable label for the final product.

The aether concentration in humans drops off significantly from infancy through puberty before stabilizing in adulthood. The Karagond Theurges pride themselves on their refusal (by and large) to make use of that fact; you've got to have something to point to that makes you feel less of a monster at the end of the day. Of course, they can't overtly explain to the laity just why they're so proud of eschewing child-murder, but they spare no opportunity to blast the Halassurqs for it. (Halassur, meanwhile, justifies the opposite choice and abhors the nightmare slave system of the Thaumatarchy.)

Aether naturally occurs on the highest plane of the cosmos, and is supposedly the substance of which the Angels themselves are made. Small amounts periodically fall to earth embedded in metal, in theory because Xaos hurls earthy missiles to disrupt the purity of the Angels’ perfectly ordered home. (The use of "sublimation" to describe aether's rapid ascent, even when it's from a liquid like blood and could thus be more accurately described as evaporation, stems from the fact that the alchemists first studied aether in this kind of solid metallic matrix.) Talismans are wrought from meteoric aether, which lasts much longer than blood-borne aether.

What does it mean that human beings are the only known sublunary source of the element that makes telos malleable? Well, some--notably Abhuman philosophers and seers, and some long-extinguished Karagond heretics--would suggest that this is strong evidence that humans were wrought by Theoi or Angels with a particular overarching end in mind, as a couple of folks surmised on the old WiP thread. This is of course rankest heresy, unwhisperable among Karagond Theurges.

It's also not the only explanation on offer. Nyrish Theurges continue to think any "argument from design" is bollocks, and kenonite or otherwise skeptical MCs will be free to agree.