Magic is a substance, and also the act of manipulating that substance.
The history of the practice of magic in Eludalsia is a lot like real-world chemistry; there were lots of old ways of doing it that were tested through practice and worked but didn't get the underlying theory right, while modern techniques are developed with a much better understanding of what's going on at a microscopic level, opening the door to all kinds of cool technology.
Magic in this world is generally not a thing that living beings can consciously do. Magic attaches itself to matter in various ways, and then magic reacts with other magic in various ways, and weird things happen as a result. Some plants, animals, and objects have passive magical properties that give them iron-hard bodies or repel the effects of flame or any number of other supernatural traits, but they cannot consciously direct the magic in them to perform spells. Magic, as practiced by humans, is the result of tools being fashioned from magical materials specifically to take advantage of these natural effects, and then, as history went on, to elicit unnatural ones.
As far as technology is concerned, magic is just one more force to put to work, like electricity or physics. Information on how it works is freely available, and what can be done with it is limited mostly by what materials are practically and legally available. NPCs and PCs alike can learn about and use magic, provided they can get what they need.
Practically ubiquitous: magical components used to repair household technology, magical components used in crafts or kids' experiments.
Commonly available: wands with basic enchantments, protective objects, assorted neat magical gadgets.
Less common, mostly seen in display cases or in the hands of rural farmers chasing off vermin: actual combat-ready staves and serious wands as would have been used by adventurers long ago.
-> MONSTER MAGIC
Monsters are the exception to that whole can't-consciously-control-magic thing.
Every monster type has innate magic, a range of things they can consciously make happen with magic. (The exception is beastfolk, who kind of repel magic.) They also tend to have deeper magic reserves to pull on when doing magic of any kind. These qualities give them all kinds of advantages, but also some vulnerabilities; having more magic stuck to them and more magical things going on around them opens up more possible angles for spells to give them a hard time from. (Think of how, the more complicated a computer gets, the more things can possibly go wrong with it.)
-> THE WILD
The Wild, formally/scientifically known as the "ethereal plane", is not so much a place as it is another layer of reality, occupying the same space as the normal, material world but only interacting with it in certain ways. The Wild is most thoroughly understood by those who study magic theory, because the flow of magic into and out of the material world from the Wild accounts for many laws of magic that are otherwise unexplainable, but in the popular imagination, the Wild is best known for being an alternate world that certain monster types can access and manipulate. Dreams are also connected to the Wild, though accessing one from the other can be difficult even for monsters capable of roaming the Wild.
The Wild, from the inside, resembles the material world, and places, people, and events in the material world can be observed in real time from the Wild, though the vice-versa is not true; people in the material world generally can't perceive the Wild without specialized equipment, and even then the perception is indirect and limited. Visually, the Wild has an otherworldly appearance, and features that would appear innocuous in the material world may be distorted in size or color or have additional properties; for example, enchanted objects may be very obviously so when viewed from the Wild, or features such as running water may block or hinder travel while other natural or constructed features hasten movement or make travel easier. Players can use any tropes related to the land of faerie or other mythological "otherworlds" to embellish the Wild.
The Wild is inhabited by mysterious, spirit-like presences that normally simply move through the world aimlessly, but become unpredictable and aggressive towards people who shouldn't be in the Wild, which is, with a few exceptions, everyone. The spirits are highly resistant to most forms of attack, being almost completely unbothered by physical strikes and only kind of bothered by magical assaults.
For humans, and for monster types that cannot naturally enter the Wild, it's a very dangerous place to be. Besides the aforementioned spirits, which will chase, kill, and devour most arrivals, the Wild may have a disorienting effect on unexpected visitors, who may be unable to make sense of, or perceive at all, the landmarks that allow traversal of the Wild. Being left alone and without any specialized defenses in the Wild is a near-certain death trap, and the very rare exceptions end up becoming something very different than they were when they entered in order to survive.
The monster types that can enter the Wild each interact with it and are treated by its inhabitants slightly differently. Refer to the Bestiary for details.
-> BLOOD MAGIC
Blood magic is the deepest, most mysterious, and least savory corner of the world of magic. It involves the conversion of life itself into magic, and THEORETICALLY magic back into life.
According to some, all magic is really just an extension of blood magic. The story goes sort of like this: long long ago, in the previous era of existence, gods roamed around and did things. And then they all died. There's a line of thinking that says they didn't just die - they sacrificed themselves, somehow, and that sacrifice chewing up their vital force was what put magic-the-substance into the world to begin with.
Actually there's more than one version of the story but go to the mythology and culture page for that.
Various people of questionable motives have attempted to perform blood magic over the course of history, with some claiming to have pulled it off, but no successes have been reliably verified - except those that have involved monsters.
Monsters can perform blood magic by burning their own reserves of vital energy to generate magic, with some really nice perks that are specified for each monster type in the bestiary. The downside is that burning through your vital energy absolutely sucks. It hurts, it makes you feel weird awful feelings, and it's easy to overshoot and incapacitate yourself, possibly permanently.
But! There is a way to die less while using blood magic! You just have to take in someone else's vital energy beforehand and burn that instead so you don't dip into your own. In fact, this is way, way more efficient than using your own; you get more power-per-unit from burning vital energy that didn't originate outside of your body. Enough to make it worthwhile, even.
"That sounds ominous" well yeah because taking someone else's vital energy requires consuming one of the following from them: blood, flesh, or the raw energy itself (which is jarred loose through various unsavory methods). All of these methods inflict harm. Draining vital energy from a person instead of eating their finger sounds like it's more humane, but it's not. It screws up their health something fierce, and it hurts a lot.
-> BLOOD MAGIC AND YOU
So, why bother with blood magic?
Besides generating magic - which is scientifically interesting but not of any direct benefit to anyone - it does weird and neat things to monsters. The process of converting raw life force into raw magic alters monsters' abilities, in both permanent and temporary ways.
The bestiary has information on how each monster type interacts with blood magic - which "feeding" methods they can or can't use, which methods get the most power-per-unit out of the person being fed upon, and what abilities are granted by blood magic.
Permanent upgrades set in once the monster has burned through one adult human's entire life force, or equivalent. (Keep in mind that taking vital energy is a big deal and sapping, say, an eighth of it from someone is still a lot of damage.) They are, as is implied, permanent, whether a monster does any more blood magic or not.
Sustained effects are active for as long as the monster keeps doing blood magic, at a rate of one adult human or equivalent every three months. In general, these effects will be more potent the more energy is being sapped and burned, and will grow weaker and weaker the longer the monster goes without performing blood magic, until they fade away.
The power-per-unit from burning one's own vital energy is so abysmal that a character could keep performing blood magic to the point of passing out for months and not reach the permanent upgrade threshold. Same goes for animals - they have vital energy, but monsters get very little out of burning it. Humans are the key; other monsters work too, but aren't super efficient.
Information on how to perform blood magic isn't something you can just waltz into a library and look up. The easiest place for characters to learn about it will be from the Joyous Order, either right after they were summoned to Eludalsia, or through later run-ins with the Order.
Members of the Order are very interested in having monsters perform blood magic, so they'll be absolutely thrilled to tell monstrous-looking people all about the method. They may attempt to coerce, entice, or outright kidnap and force monsters into doing so. And characters who display enthusiasm about chowing down on people for power and profit may end up with an equally-enthusiastic accomplice from their ranks...