Magic

Magi
In Godherja, the preferred terms for magic user is Magi, with the notable exception of Kathanouxa, the Wizard King. Magi is only a blanket term, as is Magic, with the more specific terms being Living Magi, Dead Magi, Mixed Magi, or their more specific/alternate names such as Blood Magi.

Magic in the Lore
Magic is entirely derived from Aersanon, being the power used to create all of Aeras which is "presumably and possibly... ...using the greater 'plane' of existence in the physical world to rearrange it." It is essentially a power that can tear holes in and rearrange reality to the desires of its user.

Before the Godherja and the 'killing of the world' that went along with it, Magic was a fundamentally different force. The world was simply more magical back then, the average human hypothetically being around Aeschraes' level of power ("Extremely Potent Magi I" which is the 14th rank of Magical Potential) and the average Aelfir having somewhere between Magical Potential rank 18 and 24. (the latter half of which is not represented ingame) The Magical humans who did not perish after the death of Aersanon soon came to find that all of their Magical knowledge was useless, as 99% of all their spells stopped working.

Human Magic
In the days of Aersanon and the Aelfir walking Aeras, humans stole all of the Magical knowledge that they acquired from the Aelfir. Following the Godherja, they had to start from scratch and develop new ways to work Magic. Humans have, however, never truly understood Magic, it being easy to compare them to monkeys who found a gun and tried their best to learn how to shoot each other with it. It's due to this that all current systems of classification with regards to Magic are not at all cut-and-dry, just being ways for the monkey to try to understand how the bullet fires without ever taking the gun apart so to speak.

Humans work Magic by way of rituals. Magic can definitely be done without rituals as they're simply an aide for casting spells, however those spells get progressively more and more uncontrollable the less preparation was done beforehand. This method of working Magic however does cause its own set of problems, namely that rituals must be followed to a T or else their effect will be completely different from what is intended. All of Kashiryan culture is essentially finding old rituals, mistranslating them/making errors with the ritual, then being lucky enough to cause something beneficial to happen and not accidentally end Aeras a few hundred years early.

Due to the fact that humans are not the most adept at Magic, they will usually work together when doing rituals so that the toxic influence of the Magic they use is spread throughout all of them instead of having one person bear it all. This also introduces its own problems, as the other people in the ritual will have to share the load between them if one of them dies. This can cause rituals to spiral out of control, ie Aeschraes being the only living Magi who was involved in the casting of the Self-Replicating Blood Magic, which is known to have gotten out of control in fantastic fashion.

Dangers of Magic
Magic is a force of change, even when it is not being channeled by a Magi. It has been aptly compared to radiation, as being in the presence of too much Magic can cause adverse effects for regular people. One way this is characterized is by the fact that the rate of cancer and mutations along the Waylines (and especially where they intersect) is very much increased, the heightened ambient magic being too much for them to handle and changing their bodies. In some places that aren't on Waylines like the Red Wastes, the ambient Magical energy is so overpowering that no human would ever think of living there unless their Magical Potential is quite high.

In addition to the beforementioned effects, specific forms of Magic have their own specific forms of drawbacks. Mixed Magi are known to go blind and people see them as 'ethereal,' as if they're only half there, and can often only remember them like they remember dreams. Dead Magi can gain a subtle green glow and tend to have increasingly pale skin and green or brown eyes, and Living Magi often have visible veins along their arms and neck while masters are know to sometimes possess red eyes

Gameplay
In gameplay terms, the player character can cast spells through the magic GUI, and needs to either have the relevant traits or a court Magi assigned to the "Cast Spells" job. Some spells don't need a target (meaning you target yourself), meanwhile others can either target specific characters or specific counties. For example, to cast the spell Summon Fog, you must first enter the Mixed Magic menu, select the spell, enter the Lands or Hostile Lands menus, and then select the specific county you wish to curse and cast the spell. Of course, you need the specific perk for the spell you wish to cast, you need to not be on cooldown for the spell, and you need enough Magic.

Magical Potential
As Magic tends to manipulate reality even when not put to use in spells, carrying large amounts of it within your body is inherently dangerous. All people can resist the effects of Magic within themselves to some degree, though either through luck or inheritance, some can carry and make use of much more than others without risk of Exposure Sickness. Casting spells requires you to have more than the natural amount of Magic in your body, thus only people with at least somewhat heightened ability to retain Magic can become Magi. The chance of inheriting Magical Potential can be increased through the relevant dynasty modifiers and perks.

Overwhelmation
Casting spells Magically exerts the caster's body, and a Magi can only handle a certain amount of magic before it causes harm to their body. Each spell casts results in Overwhelmation, and if a Magi’s Overwhelmation exceeds a certain point, they will suffer from Exposure Sickness.

Casting spells through your Court Magi directs the Overwhelmation to you both for balance reasons.

Gaining Overwhelmation:
When casting a spell, the caster (and their liege if the caster is a Court Mage) gains Overwhelmation depending on the level of the spell cast.

Losing Overwhelmation
A character will lose a certain amount of Overwhelmation each month based on their Magical Potential. As a baseline, a character will lose 0.2 Overwhelmation every month, then 0.2 per their Magical Potential rank is added onto it. Thus, a character with rank 5 Magical Potential will lose 1.2 Overwhelmation every month.

Overwhelmation Thresholds and Effects
There are three Overwhelmation thresholds: minor, moderate, and major. If a character's Overwhelmation exceeds an Overwhelmation threshold, the character will gain the appropriate rank of Exposure Sickness. Exposure Sickness will also degrade and disappear as Overwhelmation falls below the thresholds.

A character's Overwhelmation threshold depends on their Magical Potential. Additionally, the second Magic dynasty legacy will increase all Overwhelmation thresholds by 1.5x

A character with rank 5 Magical Potential (Exceptional Magi I) and without the dynasty legacy for example would need to have 3 Overwhelmation to reach Exposure Sickness and 12 Overwhelmation to reach Extreme Exposure Sickness.

Relevant Links
Magic guide by u/forkis (not updated for version 0.19): https://www.reddit.com/r/godherja/comments/l5sfaf/godherja_magic_a_basic_guide_to_the_three_schools/

Magic guide by u/Bommelom: https://docs.google.com/document/d/1cJMev2ZEUjhXOhch1QNJPYaZP30q1Ye3wtQ-ILLppz0/edit (credit to this guide for the Overwhelmation section)