Magic - Summary

Introduction

The first thing that has to be said about magic is that it always comes with a price. Due to how steep it often can be, using a spell in combat tends to be the Initiate' equivalent of a Nuclear Option. Thus, despite almost all Initiates knowing at least a single spell, you can expect most of their fights to be fought with weapons of a mundane world. A pistol or a fire axe will wound anything physical, even if it hails from another universe. And a glyph-covered dagger will do the same to non-corporeal entities.
However, a timely executed spell can be a game-changer. Especially when something drove you into a corner. And you can expect that to happen, as most supernatural entities (especially the demihumans) and cultists of the Powers are less restricted with their ability to cast magic. There is still a price to pay, but it tends to be paid in a different, more palatable (or not) way.
In most cases, you can still counter that with mundane (but well-applied) firepower.

Magic

What?

Magic, in its purest form, is theurgy. Reading correct books doesn't give you the ability to warp reality. It does, however, gave you a form of mental connection to one (or more) Powers, allowing you to use their power to warp reality. This is the main reason why using magic is incredibly dangerous - Powers, after all, are nigh incomprehensible and borderline omnipotent eldritch abominations. Any form of connection with them ISN'T safe. Quite the opposite, in fact.
Every Power has its own magical 'domain'. Seeing those as a 'school of magic' straight from a fantasy world is a massive oversimplification, however. Every spell is a minuscule shard of the totality that its Power is. Casting a stereotypical fireball through the power of the Primordial of Fire isn't 'simply' borrowing its power to make the flame. Primordial of Fire's nature is that of an all-encompassing flame (probably the size of the universe). To cast a fireball is to carve a minuscule part of this nature (the part that's distinctly fireball-shaped) and unleash it upon your enemy.
Casting even the simplest spell is to expose yourself to things that are fundamentally beyond human understanding. This is the very foundation of the 'price' that mages have to pay to use their craft. In fact, the most powerful of human mages tend to use their spells the least. They still have things to lose, and thus can do the most when truly cornered by something.
The exception to that system are the glyphs. No one truly knows what is powering them. Theories range from One True God through Humanity's subconscious will to a long-forgotten magical construct siphoning the energies of the Powers. They are cast externally, by carving or painting correct shapes onto non-organic materials and thus cause no risk of 'side-effects' normally accompanying magic. They also seem to carry a deeper than normal connection with the equally mysterious Censor's Veil.

How?

The first step of casting a spell is learning it. To learn a spell one has to connect themselves to the Powers. This is achieved either through correct sacrificial offerings, exposing yourself to artifacts of power or by reading books, scrolls or tablets soaked in their power. The last way tends to be used the most, as it is the most predictable. The same book (for example a diary noted down by a frenzied cultist of some Power) will teach different people the same spell, while demanding the same price. Artifacts are are less predictable, and offerings and rituals tend to depend entirely on the mood (if they have those) of the Power in question.
To learn a spell is to expose yourself to the Power. This WILL damage the mage's sanity and corporeal stability. Even if the Power they connected to appears to be vaguely benevolent. To be surprised by the latter is to be also surprised that gravity sometime kills peoples, despite helping the rest of them not fly out of atmosphere into the deadly space beyond. Sure, in case of the weaker magicks of non-malevolent Powers the damages tend to not be permanent. But once you progress from your infancy as a mage, you better get ready to trade yourself for the spells that you need.
Simpler spells are casted simply at will, while the stronger ones sometimes require a vocal component, certain gestures or, in a more radical situation, offerings to be properly executed. The moment a spell is learned, one will always know how to use it, basically speaking through instinct, the spell itself becoming a part of their nature.

Price

There are several ways in which using magic harms the user. The most common ones are sanity damage, corporeal damage, physical harm, gaining attention, and permanent loss of humanity.
Sanity Damage is the result of your mind getting exposed to things that it wasn't able to bear, i.e. the Powers and some choice other supernatural entities. It shouldn't be imagined as 'mundane' insanity, although those two carry some connections. In a bit crude terms: if the human mind was a dishwasher, then the normal insanity would be a malfunction or a design flaw while sanity damage is best compared to someone hitting it with a sledgehammer. Or dousing it with gasoline and throwing a lit match.
Having your sanity damaged is something that every human feels in a bit different, personal way. The most common result of your sanity weakening is a gradual decline of your ability to differentiate what's real from your imaginations and delusions.
The human mind is capable of recovering from a lot of things. Sufficiently nasty sanity damage is permanent, but otherwise, you'll eventually recover. The problem is that human minds were designed to deal with design flaws and malfunctions, and not with sledgehammers. As a result, it's pretty common for them to heal wrongly. The results are behavioural changes, often consistent with various mundane mental disorders.
This is a roll of dice, however. Negative changes are just as likely to occur as positive and neutral.
Corporeal Damage is the result of your body being exposed to energies that it wasn't designed to bear. In the most simple of terms, if you channel too much energy of a Power through your body, you may start to physically resemble it. The results are mutations, or various supernatural afflictions (often summarized under the word of 'curse').
Once again, like with sanity, your body can and WILL resist the changes. The mutations will be either minor or will revert to their natural state when given enough time. However, if your corporeal stability is either overwhelmed at once or not given time to recover after your mutation manifested, your body might begin to consider the altered state of itself to be its normalcy. When that happens, reverting the changes is extremely hard. And might not be worth the price.
Mutations, just as mental changes caused by damages to sanity, can be helpful. The problem is, they are random (although there is a relatively cohesive set of changes per Power in question). The Censor's Veil will, naturally, hide them from the non-Initiates. For many Initiates, however, visible mutations are a warning sign. It's often a mark of cultists or simply reckless and suicidal behaviour.
Physical Harm is perhaps the most mundane of the threats that magic brings, but it's a threat nonetheless. Launching a burst of fire from your hand might be a good way of pushing an attacker back, but it's not exactly nice for your hands. Overusing it will result in burns. There are certain spells that, when overused, might kill you. In the most mundane way possible.
The biggest problem with this type of feedback is that it tends to be surprisingly hard to fix. Weaker sanity damage will fully restore itself in a few days, however, a burn might cause permanent scarring. Worst of all, what appears to be a simple wound out in the field might get infected and kill you nonetheless. Obtaining the ability to resist your fire tends to not be worth the price, and healing your self-inflicted injuries with magic only forces you to deal with other forms of damage.
In a positive twist of fate, magic that causes a notable risk of physical harm (especially that of Primordials) tends to be relatively light on sanity and corporal stability. Of course, you still have to bear the pain and potentially life-threatening injuries.
Gaining Attention is a slow and insidious killer. Powers are de facto gods, for whom humans are akin to ants. Unimportant and small things existing in the background and rarely paid attention to. The problem is that if an ant shows up on your desk and somehow manages to push a button on your computer's keyboard, you will notice its existence. It is possible (when you are very careful) to avoid this fate entirely, but most Initiates have managed to make at least one Power realize that they exist (or, in worst case scenario, more than that).
Obtaining at least some of said attention is simply inevitable, for as long as one wishes to learn more powerful magic. In fact, obtaining it empowers your magic of that particular Power. However, as stated, there are risks involved. If you keep gaining it, expect things to start turning bad. Supernatural entities will be more prevalent in your life. Your magic will be more powerful (but also more likely to blow up in your face). Results of sanity and corporeal damage will become more 'personal'.
If it goes high enough, the Power might start making you do things. Expect cryptic and sanity-damaging dreams, unexpected coincidences and the Reality itself appearing to railroad you in certain activities. This is best compared to a human using a ruler to 'guide' the ant to a particular part of his desk. Why do they do that? You will not know until it is too late to get out. It's a high-risk high-reward situation, as you can obtain great powers and powerful artefacts... or you'll end up wishing that you were dead.
The Attention typically decreases over time when the situation isn't worsened, but it's not always the case.
Total Loss of Humanity is one of the few eventual outcomes of Initiates' life. Death is there (either violent or, if they are good enough, from old age). Madness is there. And, finally, there is a threat of no longer being recognized as a human being. By the Censor, by other humans - and finally by yourself. The exact mechanism of this phenomenon varies. Basically speaking, there are three main outcomes.
First, is become an undead. This means dying, but not entirely and, as a result of something - overbearing emotion, dying in the wrong place at the wrong time, some magical ritual - continuing to exist in a permanently altered state.
Second, is to fall, becoming a demihuman (or a Fallen, Lost or one of the numerous local terms for them). This typically happens when you obtained excess attention of a Power that decided to 'refurbish' you in the way that they saw fit. Most of the Powers have at least a singular demihuman species connected to them. Unlike simple mutations, those alter your very nature, meaning that from now on, NOT being human will feel natural to you. The chances of reverting the change are thus almost non-existent.
The third, extremely rare case, is apotheosis, which means becoming a form of aberrant. It might be forced (in which case it's typically a form of a fate worse than death), or, in rare cases, it's actually someone's goal.