How Magic Works
Magic ability for humans is embodied in the Tao stat (or whatever it is called by different cultures). The Tao stat is always divided up into three substats like the other stats.
Northern Tao stats are Life, Death, Blood.
- Necromancy is Life and Death. 
- Beast Mastery is Life and Blood. 
- Battle is Death and Blood. 
Southern Tao stats are Light, Dark, and ???. (I have much less info here)
- Shadow is Light and Dark?
- Southern magic seems tied to their gods
I don't know enough about Witchcraft and Demon Summoning to know if they are just specializations or variants of magic systems or elements that have parallels in all or some magic systems, but I suspect that they have local parallels in most or all magic systems (although they may not necessarily be practiced due to local laws/customs, etc)
Evidence for why I believe magic is all tied together:
- "Yang Shen-Ji believes that corruption points seem to be an attempt (or a measure of success of an attempt) to tie someone's chi to the barbarian lands of the north, and to the rules of that land." If one can tie someone's Chi to the north (or south), they must be related or parallel in some way; linking totally disconnected stats doesn't make any sense. Also, I've had both types of corruption points personally and have examined Shohag, who has/had Imperial corruption points. I don't think this is written down any place, but Mike told me that reading the notes of Mu the Mad had indicated that I could transform my stats into Northern stat if I got enough corruption.
- Mola Ram has been conducting rituals that have been familiar in many ways (invoking the Tao, invoking elements), but has been doing it in a way that I have found not right . This suggests that either southern ritual magic is similar enough for Mola Ram to warp/adapt our magic rituals to his purpose OR to use his rituals in a warped way to affect our magic. Either way, it suggests a connection or parallel between the two.
- "Hana also experiences some changes - she finds that she has Life/Death/Blood instead of the normal Tao stats (and her Death is substantial)." . The fact that she swap stats suggests a strong underlying connection.
Evidence for what might be termed local flavors or variants that distinguish the types of magic from each other:
- From Life in the North: "Qutugh himself is somewhat Yang-ish, but his Yang has too much Yin in it, which Takanata doesn't like. As both Takanata and Qutugh take (opposite) corruption points from crossing the streams with each other, Takanata notes that that bit of Qutugh's Yang sorted itself out better to become more properly Yang-ish."
- Also from Life in the North: "They have no equivalent to sorcery, which uses all three Tao stats - there are masters and lords of Life, Blood, and Death, and also of all the pairwise combinations"
The conclusion here is that while Tao is always divided into three, the ways the substats can be combined/used in any particular magical system are one of the defining characteristics of that system. Another major defining character is likely the basic characterization of the individual stats; a Death substat is only appropriate for a culture that has a strong connection/feeling about Death and vice versa. It's not clear to me whether the culture defines the substat or vice versa though.
Int roll with all relevant magic-type skills: 12 successes (7 fortune thrown in)
- This info comes from talking with the Life Master during the Life in the North run; a couple of these are confirmed by the Connections Reading during Information Hunt
- Specific necromancy info from fighting necromancers more times than I can count, the Diary of Mu the Mad, multiple necromantic items, talking with Shohag who also had KS: Necromancy
- Beast Mastery info from seeing the Beast Master during Winter Has Come, plus examining the wolf amulet
- Battle Magic info from facing the Battle Mage during Winter Has Come, looking at his staff afterwards
- We have a mixed bag of info about the Southern gods from Chochiro, the spirits in Dutiful Serenity, and Iala Mane.
- Corruption Points wiki page; I can't find the email.
- My direct observations of the ritual remnants near the Southern Wall as well as various elements in several runs (the cage to capture the Jasmine River, etc)
- From Life in the North