For context, I draw mostly upon the 6-8th Edition books for the tabletop wargame, secondarily on the 2nd Edition of Warhammer Fantasy Roleplay. I don’t really think the End Times is good writing so I tend to mostly ignore it, and haven’t read any novels, so again they’ll be pretty summarily ignored. Think of my answers as describing the world in the state it was before everything went to hell with the End Times.
My primary interest is the magic of the world but I’ve gone through just about all of the sources I’ve listed above with enthusiastic obsession, so ask anything. Here’s an example explanation of how magic works in the Warhammer world.
The Winds of Magic blow in the Eight Winds from the ruined Gates of Heaven on the poles. They’re in a sort of a messy jumble. To sculpt that messy jumble into spells via raw willpower is what’s known as Dark Magic, and prolonged use of it for any substantial feats of magic is corrosive to any living creature’s sanity.
The Elves, and the Slann who taught them, were created by the Old Ones to be naturally magical. As such, Elves aren’t unduly psychologically affected by the magic they wield, though Dark Magic’s corruption will eventually claim them as well. The Slann taught the High Elves safe methods of working magic - extracting one of the eight Winds from the jumble, and purifying it, then using these single purified Winds of Magic to work magic of a certain character (since each of the Winds has a character of its own), and then weaving the purified strands back together into something called High Magic, that could be employed to do things one Wind couldn’t alone.
The Empire’s eight Colleges of Magic practice the High Elven system, but with one difference: Humans are disallowed by law from studying more than a single Wind and its associated Lore.
Why? Humans happen to be pretty good at magic, but they weren’t made by the Old Ones to be mages. The end result is that Humans are much more psychologically affected by the magic they wield than Elves are. For example, human students of the Lore of Beasts grow increasingly antisocial and misanthropic, and essentially all end up leaving civilization behind to live in the untamed wilds where all the other Humans they see are random hunters and their fellow Beast shamans. The Golden Order who study the Lore of Metal grow obsessed with studying and analyzing everything, and their Order works as much on studying things like alchemy, metallurgy, forging, herbalism and the like as sorcery and runesmithing proper. Every one of the eight Winds has a personality of its own, which is imparted on Human masters over their years of study, leaving every master an eccentric.
Now let’s imagine what would happen to someone who heavily used both Ghur, the Brown Wind of Beasts, and Chamon, the Yellow Wind of Metal. A studiously analytical, pragmatic scientist who’s uneasy in the trappings of civilization and kinda wants to vibe with the spirits of wild animals? It’s easy to see how one would end up with contradictory influences and end up insane, which is rather against the whole point of the Colleges and the system studied there.
Priests are shielded from the changes wrought by magic by their patron god. Their divine powers are still spells that shape the Winds to achieve the priest’s and god’s ends, but the Winds are shaped by the priest’s faith, the god’s dedication to their servant, and the religious rituals and prayers shaping the pair’s expectations of what should happen. Elven Mages, Imperial Magisters and the Liche Priests of ancient Nehekhara shape the Winds into desired forms through technique and ritual, practicioners of Dark Magic (Chaos Sorcerers, Skaven wizards, Necromancers, random untrained practicioners of magic) largely with sheer will, though structure and ritual play a large part in Necromantic magic, the Nehekharan system is in part religious, and the Chaos Sorcerers’ works are in part worship of their Daemon Gods.
The Mages and Priests of the High Elves and the Nehekharan Mortuary Cult are a good bit like Chaos Sorcerers, Grey Seers in that they work the more wizardly sort of magic and are technicians, not pure worshippers, but the Nehekharans are literal priests of their gods, and the High Elves’ Mages and Priests are both, well, mages and priests.
The Mages study magic as a technical endeavour but the attitude towards the study is partly religious and spiritual, and the Priests just lean more into that. Among the Imperial Orders, the High Elves’ attitude would most closely be matched by the Light Order but the vocations of priest and wizard are much more distinct in the Empire than they are in Ulthuan even though both study the same system of magic.
Kerillian’s mutations are not caused by her practice of magic - Wood Elves don’t use the same system as the High Elves and Imperial Magisters, but do have their own safety measures. Kerillian’s physical changes are due to her joining a religious cult that takes part of the new inductee with it and replaces it with something… different.
Sienna’s tattoos, likewise, are attuned to her magic, but are not caused by it. The way they writhe and how her hair sets itself on fire when she works magic are caused by the influence of the Red Wind on its Human practicioners.