Kor-Ithar
The Kor-Ithar is the reserves of magical energy that resides inside every living being. It makes creatures able to use magic by tapping into it, yet a majority have to learn how to use it.
The potential of the Kor-Ithar is how much energy it can store. This is not a set amount since the potential will increase if it's used regularly, just like exercising a muscle. It will also decrease if not used for a really long time, but not as fast as a muscle, and never below it's initial potential.
Using the Kor-Ithar
Scholars studying the Kor-Ithar say there are two types of beings in the world - those who can use the Kor-Ithar naturally, and those who have to learn how to use it. Natural users among the humanoids are called Sorcerers, while those who have to learn how to use the Kor-Ithar can have many names, partially depending on how they have learned to use it. Among these are the Bards and the Wizards.
It takes years learning how to use the Kor-Ithar, and even the Sorcerers will usually need to learn how to control it, either through practice or through lessons. Using it is complicated, and regulating the power of the energy as it is being sent out of the caster's body takes a lot of practice to get right.
The best place to learn how to use magic is at the Academy of the Arcane in Lenthir, which is an educational institute specifically meant to learn how to use the Kor-Ithar for wizards and sorcerers. The enrollment fee is steep, so the education is generally limited to the more well-off people. Bards will have to go to the Minstrel's University to learn the skill, but getting enrolled here is extremely hard since only a handful of new students are accepted every year, most of which will drop out due to the highly competitive culture among the students.
Running Out
Almost all magic users who actively use magic will sooner or later experience the feeling of running out of their magical reserves. When the Kor-Ithar is empty, it will take time to replenish the magic energy to the ordinary level. It will replenish slowly as long as it isn't in use, and faster during rest. A good rest is without physical or mental strain, and without alcohol or other similar influences.
It is pretty easy for a trained magic user to know how much energy they have left in their Kor-Ithar. If they tap it slightly with a tiny useless spell they can tell by the slight feeling of reluctance from the Kor-Ithar itself as it starts to be drained. How this reluctance feels is individual from caster to caster, so this skill is only acquired through experience.
A magic user can keep using magic even when their Kor-Ithar is drained. Magic will then use the energy of their life force, which is vital for survival and much harder to replenish. The magic user will become physically and mentally weaker as their life force levels are used, and it is lethal to use too much. Using life force also comes with a significant level of pain. This is generally only done in life or death situations, as it will render the caster feeling weak and sluggish for days to weeks, depending on how much life force is used. A magic user can never accidentally start spending their life force - it is a conscious decision. Still, it's done in much the same way as pulling energy from the Kor-Ithar, so the majority of magic users will intrinsically know how to use life force.
Sensing the Kor-Ithar in Others
It's possible to sense the potential of the Kor-Ithar of other people by using the right magical technique. It's finicky to do right, so it's only the more experienced magic users who can make it work. It's also quite energy consuming so it's not done often, but in some situations it might come in handy. Most of the faculty of the Academy of the Arcane knows how to do this, since it might be helpful when giving guidance to students.
It's also possible to hide your own Kor-Ithar potential from detection. This is a skill that's hard to do, and it's even harder to find someone that will teach you how it's done. This is considered slightly rude among magic users, so there's no instructions to be found in any books commonly used to learn magic techniques.
Kor-Ithari Stones
A Kor-Ithari Stone is a crystallized piece of Kor-Ithar. Although very rare, these crystals are used in magic items to give them power. The color of the magical energy of the type of magic will decide the color of the Kor-Ithari Stone.
For a second there I thought you were going to make a binary joke :D Good article. It makes magic use sound quite scary. Or whether burning your own supplies. Is this considered a forbidden art? Or restricted?
It's not forbidden, and the only restriction is that the needed education to be able to use magic is expensive and only available at some very few, and in some cases very exclusive educational establishments. Magic in itself is only reliable when you know exactly what you're doing, and using magic without knowing how to control and use it is always dangerous.
Thank you so much for the comment, tho! <3