This is an old revision of the document!
Magic is a rediscovered art. The setting’s equivalent of the Aztecs basically discovered it and so archeology is a useful field of study for sorcerers, occasionally discovering new spells just by making sense of some old writing.
In-character, it’s still unclear how the Aztec-equivalents originally discovered it at all, but the answer is basically fucked up human sacrifice rituals letting them (coarsely) communicate with the Aether that underlies this universe, which, depending on how you look at it, is either alive or consists of a million Eldritch souls suckling on the universe as a whole, possibly being the cause of entropy (the author hasn't thought much about this aspect).
Basically, fleshrunes are the quickest, least deadly way of ‘communicating’ with these things, and you may freely consider these like instructions to a computer, if the computer was an unspeakable Eldritch god that sometimes instead decided to reach out and vaporise you instead, because you accidentally spelt out an insult.
To clarify, this is a flippant summary, the Aether doesn’t get offended. But it may for any one reason decide it’s not in the mood for instructions. It’s an extremely rare event for spellcasting that is done right, but sorcerers are definitely playing with fire even if they’re doing everything optimally. The Aether can and will eat you alive – literally – if ‘provoked’. The chance that you’re provoking the Aether rises with spell tier and with each failure.
Also, if you do ask the Aether to change you in some beneficial way, don’t expect that to work out entirely as you expect. You’re asking for a swarm of invisible monsters to enter your body and change things. Expect to go a little insane (or rather, a little alien – your ability to function and reason about things will probably not suffer). See again the comment about Hzataalar.