Magic

Ambient Energy

There are a number of magical energy sources in the world, thought to be related to the gods, with each source of energy having a different function. However, the gods cannot give or take access to these magical energies, they are simply the source of the energy. Their power emanates from them into the environment as ambient energy, and those with the right knowledge can tap into these energies. Seemingly, the gods are not aware when their ambient energy is tapped into.
Which god grants which power is not entirely clear, some are quite obvious, being directly related to their primary aspects or personality, while others are more subtly related to them.
Because the gods are not able to control people's use of their energy, it is not influenced by worship, faith, or dedication to the gods. Rather, they are tapped into by intent, achieving the right state of mind and emotional motivation for the energy to sync up with you and flow through your body.

The Energies

There are thirteen different energies, ten of them are light, three are dark, one for each of the Velen and Forsaken respectively. Each of these energies requires a different state of mind to be tapped into. In matters of attributes and requirements, some of the energies overlap somewhat, while others are unique.

Those Who Find the Power

Mages are extraordinarily rare, not because the power is hidden from most people, available only to a select few, but because the secrets of how to actually tap into these magical energies is a complicated matter. Even those who have studied magic for their entire life, knowing all the theory, may find that actually tapping into the energy proves to be another matter entirely. Among those who achieve such a thing, many may never fully master the skill, failing to attain a proper state of mind. It is because of this that mages are rare, and strong mages are rarer still.

Conduits

Magical energy is a volatile power, it is drawn from the environment into the body, where it is used when possible. The caster can then direct it outwards into the world in the form of a spell. By default, magic can only leave the body through natural holes, such as pores, the mouth, or the nose, but this isn't the most practical for the casting of spells. This is why mages might use a conduit, a device that can be used to cast spells. This is a magically attuned object, usually quite small, that is worn in such a way that it can easily direct spells outward, such as a large pendant worn at the breast, or more commonly, a glove, with the conduit in the palm of the hand.
Magically attuned objects are rare and can only be crafted by the use of magical construction and deconstruction. A mage will have to construct a mineral that is both tough enough to withstand the great force that will travel through it and that contains the proper magically conductive elements so they can actually be used as a conduit.

The Toll

Each different energy has a different effect on the human body. However, what all magic has in common is the toll it takes on living beings. Those who cast without care, or without heeding the warnings will quickly find themselves suffering from one of several magical maladies. They usually affect the parts of the body that the mage uses the most, such as the arms and torso, but this may depend on the type of energy.
There are also ways to partially revert or circumvent these ailments, but that may leave one unable to cast magic or do much else, since everything has a cost.
The most common of these magical maladies is called split organ syndrome, which occurs when repeated magical activity damages one or more of the caster's internal organs. This may require extensive healing or the removal of an organ damaged beyond repair, in many such cases, a transplant might be required.
All casters will bruize themselves while learning to cast correctly, and this isn't immediately disastrous. It is the repeated magical bruising that causes these magical maladies.

Energy Transfer

The toll is a result of the energy that travels through the body, causing it to wear down over time. The primary way these energies are transferred throughout the body is via the nervous system, which is naturally resistant to this. However, for someone who has never cast magic before, their body needs time to adjust to it. Beginning mages suffer magical bruizing faster, and they have to be extra careful to avoid injury. Over time, the nerves develop a thick outer layer, a type of callous, that increases their resistance. It will still wear the body down over time, but mages who have been careful and have been casting for a longer time will be able to cast much more and much more efficiently without suffering as much from it.
Magical injuries are caused by mages overestimating their resistance to the magical forces that rampage through their body on their way out, accidentally hurting themselves. It is impossible to completely avoid getting hurt, but the careful and those who show restraint always live longer.

Perception

Public perception of mages varies greatly throughout the world. Some people look up to their rare mystical abilities, eager to hear stories of their exploits or their battles with dark forces and their knowledge of the world. Others look onto them with suspicion, as their conviction may be that such a rare and powerful ability cannot possibly be obtained in any way to be considered "natural". They might avoid mages, encouraged further by the spreading of eerie stories and dark rumors of mages and their activities. Others still might hate mages with a passion, blaming them for the wrongs in the world, naming the hijacking of the gods' ambient power a vile abomination, punishable only by death.
This is also why, depending on the region, a mage might choose to neglect wearing their conduit or conceal it under their clothes, while in other parts of the world, mages might wear them open and proud, a symbol of their status and power.
Continue reading about magic in the article: Magical Energies

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Healing

It should be mentioned that magical healing, while powerful, has a very limited capacity for healing magical ailments, and it isn't as easy as just casting consumption every time you split a lung.

The Stretching

While being a mage poses a significant threat to your personal health, those who practice magic, and have done so consistently will find their process of aging to slow down. It will not stop completely, and your body will age in other ways, but age is not likely to kill mages, and if they are capable of managing their magical symptoms, they might still live a lot longer than the average person.

Contagion

In addition to magical maladies, casters also need to be aware of the contagion, a foul force that has polluted the magical energies, and any wise mage will do their best to avoid it.
Read more about the contagion in the article about  Aetherium.