Keirsi is the inherent magical energy that makes up Arvastaira. It is found in anything and everything. In fact, it would be quite alarming if there was a void of keirsi anywhere, no matter how small.
Even in the beginning, when there was nothing, there was keirsi. Atei himself is also made of keirsi, existing as the sole entity, living or otherwise, with such an incredibly dense composition of it. Generally, the denser the keirsi in the soul, the more powerful something is.
In its most natural, dispersed state, keirsi is fluid and malleable. Things have the form they possess due to the specific frequency and shape keirsi takes on under certain conditions. For example, rocks and minerals will have a keirsi structure wholly different from wood. Even different types of rocks and minerals will have different structures. As a further example, when the frequency and shape of a flint piece strikes a steel piece with enough force, it distorts the formation of the keirsi and changes some of it into the formation of a fire spark.
Keirsi will replenish itself over time naturally. Rarely will something lose all of its keirsi: otherwise, it would cease to exist. The timeframe it takes for keirsi to regenerate varies greatly, so there is no universal duration. It is noted that during the month of Fonsertinna, it does tend to regenerate a little faster on average, compared to its counterpart month, Kafeltinna, where it renegerates a little slower.
Each being has a natural cap to how much keirsi they can handle at once. This is the limit they are born with as an individual. With dedication and training, this natural cap can be slowly overcome. However, any entity that is not divine will have a true limit that is shared by all non divine beings. This is a limit that is seldom overcome and one that few creatures ever get close to.
Developing skills related to keirsi comes in two categories: limitations and manipulation. The former is the amount of keirsi your body can contain or work with at once. This can be enhanced through meditative exercises where one draws into a natural reserve of keirsi and works with it repeatedly. The latter can be enhanced through repetitive casting drills and precision focused exercises. How quickly one can improve depends wholly on the individual in question. A good balance of both limitation and manipulation training is necessary in order to achieve greater heights.
One's development of keirsi skills can also fade over time. It's comparable to working with any muscle in your body. For someone who has reached high levels of skill, it will be easier for them to return to that state compared to someone who has never been there before. But if an individual does not work with keirsi for months, they will find that their limitation and manipulation abilities have changed for the worse.
As a side note, the way individuals interact with keirsi can drastically be altered depending on their emotional or mental state. The more prominent and unstable the state, the more powerful and unpredictable keirsi manipulation attempts will be, often causing results that would otherwise be inexplicable for other entities of the same caliber.
To the naked eye, keirsi structure and other attributes are mostly imperceivable for anyone except the divine. With the assistance of various tools, fenva and other entities can examine the traits of any specific group of keirsi. However, there are some things that most tools cannot even help one see.
Some individuals are born with a series of mutations called vutansen that leaves them more sensitive to keirsi. This trait comes in a wide spectrum. On the less prominent end, it includes traits such as being able to intuit the differences in keirsi concentration within rooms.
On the most prominent end, these individuals, known as yannhara, are incredibly sensitive to any minute fluctuations in keirsi. Many of these yannhara are able to control their senses and prevent themselves from being overwhelmed, but they are always aware of the state of surrounding keirsi, akin to seeing it out of the corner of their eye. Most yannhara also grow into highly skilled sorcerers.
Yannhara can be very good at reading people and estimating how skilled a sorcerer could be. This is because unless one is highly skilled in the art of keirsi manipulation, it's difficult to obscure the characteristics of one's natural keirsi. However, reading others' keirsi can also be wildly inaccurate due to all the factors that come with it.
Reading someone else's keirsi signature is a strange sensation. It produces a feeling in the yannhara's chest, deep inside their flesh. This feeling, though not directly affected by the environment of the yannhara, comes off as a combination of movement, pressure, and temperature. Higher pressure corresponds with greater capabilities, though most individuals with great capabilities regarding keirsi will intentionally mask it, making the likelihood of attaining lower readings much more common and inaccurate. The movement sensation dictates the manner in which this individual's keirsi often manifests, usually giving a clue about their disposition. For example, someone with high speeds of linear movement might imply a manifestation of light, wind, or that the sorcerer themselves fights with a focus on dexterity and nimbleness. This movement is almost impossible to mask, though the intensity of the movement one projects is definitely alterable with practise.
Finally, the temperature dictates the way one's keirsi manipulation is aligned. The warmer the keirsi, the more the individual aligns with vesera magic techniques. The colder the keirsi, the more the individual aligns with lumena magic techniques.
The two major factions in Arvastaira practise the art of keirsi manipulation very differently. Though not always decided by the faction one is born in at birth, it heavily influences one's innate preferences and capabilities. Most of the time, individuals are very heavily skewed one way or the other. Lumena aligned individuals who can put on great displays of sorcery may be unable to so much as light a small flame when using vesera techniques, and the opposite applies as well.
Those aligned towards lumena magic are most well attuned to techniques that draw keirsi out of their own forms. As such, most practitioners of lumena magic will have great capacity for keirsi presence within themselves. In a way, they are at a natural surplus for internal keirsi compared to vesera practitioners. The keirsi they have within themselves also regenerates far more rapidly than their vesera counterparts, allowing them to draw from their own stores continuously throughout a day. However, they struggle to draw keirsi from any source but themselves.
Utilising lumena casting techniques can come with certain benign side effects occasionally. Lumena casting often makes the caster feel a little cold or chilly. It will feel like you are reaching within the depths of yourself and forcing your will into reality. However, many of the sensations associated with lumena techniques also depend on what the keirsi is being made to do within the context of the individual caster. Sensations are usually consistent between sorcerers.
For example, if one uses lumena casting to manifest a small flame, it could cause them to feel a little cold, as if the temperature dropped. If they were to create a large explosion, it might cause a chill to run down their spine instead, the cold being magnified by the extent of the feat. However, in this same instance for a different individual, it could cause the heat to physically leave their body instead. Creating a small flame might cause their index finger to feel cold, whereas a large explosion could cause a cold to emanate through their entire arm.
Those aligned towards vesera magic are most well attuned to techniques that draw keirsi out of their surroundings. As such, most practitioners of vesera magic will be highly skilled in manipulating keirsi. They have a natural deficit for internal keirsi compared to lumena practitioners, along with a slower regeneration rate. Keirsi can be drawn out of anything around them in their affinity range that does not have a will to resist. However, they cannot draw out all of the keirsi, meaning there is a limit to how much they can take and the rate at which they can take it. It is possible and not uncommon to take enough to alter the state of the original source, such as taking so much keirsi from a plant that it withers.
Unlike lumena casting, which focuses slightly more on stamina and consistency, vesper casting has a focus on quick bursts and variation. Vesera users often bring trinkets or objects that hold large amounts of keirsi in the event they cannot draw from their environments when needed. Much careful planning is needed to make sure one has enough keirsi for an excursion. However, if one does not have enough, more adaptability is necessary and the need to think on your feet to obtain that keirsi is vital. Because of this caveat, vesper casters are highly skilled in manipulating keirsi, as every little bit matters with their more finite resources.
Utilising vesera casting techniques comes with their own benign side effects. This often manifests as warmth or heat, as if the temperature is rising. It will feel like you are connected to the target you draw keirsi from and you are coaxing it to obey your wishes. For example, casting a small flame as a vesera user may cause your body temperature to rise. Casting an explosion may make you start sweating profusely. For a different person, a small flame could cause their fingertips to tingle with warmth while a larger explosion could cause their arm to feel as if it had been singed.
The neveniati, colloquially referred to as neven, are able to easily utilise both Vesper and Lumen techniques. They often possess enormous potential for destruction. Neveniati are haracterised by a huge surplus of internal keirsi alongside a simultaneous natural conductivity for keirsi in their surroundings. However, this does not always mean they are able to manage the sheer amount of keirsi that they are saddled with. If they lose control, which happens to many, the results could be disastrous. As such, they have a generally short average lifespan. Either they will destroy themselves or others will seek to destroy them as a precaution.
The sensations associated with neveniati casting depends on what they draw from. If they utilise only either lumena or vesera casting, then the sensations will be similar to their respective techniques' typical effects. However, if they choose to utilise their innate abilities and simultaneously draw from both themselves and their surroundings, they will feel almost nothing at all. It is said to feel effortless, freeing, even pleasant to do so. This makes neveniati techniques all the more dangerous, as the individual rarely sees they are pushing themselves too far.
Harnessing or interacting with too much keirsi, more than you can handle, can have a wide range of consequences. Some may feel exhausted, sluggish, experience brain fog, or even black out. Others may feel a deep discomfort within themselves that ranges from an ache to a sharp sting to a pain so severe it will send the caster into a state of shock. Though most cases of keirsi manipulation, especially sorceries, are only temporary in the world, overexertion or misusage can definitely induce lasting mental or physical damage to the caster.
Coming into contact with too much keirsi can go so far as to kill a person. Because of this, the arva do not manifest in their original draconic forms, as the sheer amount of keirsi they exude when draconic is enough to instantly annihilate most things within a certain distance.
Loosely known as "magical composition", nantak is a product of the soul and the keirsi it binds to. If one thinks of the soul as an anchor weighing down the keirsi that forms the body, the nantak is like a naturally formed net encompassing the perimeter that reigns the keirsi in, containing it and giving it further structure. The ambient keirsi around a soul has its own natural shape, forming the non magical aspects of an entity: their mind, their body, their voice, their personality.
Nantak simply molds the keirsi more precisely and with greater detail, playing a large role in determining what magical characteristics one has. These include traits such as the kind of casting techniques they are most comfortable with, their affinity, and their proficiency. Nantak itseelf, is also comprised of keirsi, but it is far denser than the keirsi used to form the body, though not as dense as the soul itself.
The soul (also referred to as nauhas) of any living, sentient entity is a strange combination of keirsi only truly understood by Mirnat and Atei. Souls all have unique frequencies and shapes that are far more complex than the structure of most other keirsi. Depending on the structure of the soul, it will alter the keirsi it draws in too. In a sense, souls are a beacon and incubator for keirsi and serve as the core for all entities in Arvastaira.
When an individual life is made, the soul that comes with it draws in keirsi through the mother's body and slowly alters its qualities over time. At a certain point, that keirsi becomes recognisiable as a living being. Over the course of one's life, the soul will continuously alter the keirsi that it has claimed as its own, causing growth, aging, and eventually, death.
Death occurs when the keirsi surrounding one's soul is ripped too far away or becomes too depleted. The soul has a tight hold on the keirsi it claimed, having bonded with it intensely, but on its own, it has no way to regenerate that keirsi in the same structure.
Yet when one dies in Arvastaira, they do not truly die. The soul is released back to Mirnat and only he will decide when it can be released once more. An identical soul does not always create an identical new individual. The way souls draw in and alter keirsi during the making of life can widely vary depending on nearly countless factors. But it is also not entirely unheard of for someone to be reborn with some traits that are identical to their past self's.
To provide an example, a specific soul might naturally draw in and morph keirsi to create a brave individual. It may create a war general, a selfless martyr, or an individual that cannot feel fear. If a selfless martyr was created, there is a non zero probability that the next cycle of life will also be that of another selfless martyr with similar amounts of bravery.
Over the course of one's current incarnated life, they tend to reconnect with the memories and experiences of their past one. However, it is an extremely gradual process, and one that does not truly complete until the moment of death. Mirnat is also the only one who can create new souls. He can also break apart and merge souls if he sees fit, but whether he can truly destroy every trace of a soul is unknown.
Unlike all other living, sentient entities in Arvastaira, fenvasi do not have a proper soul. It is still referred to as a soul by most, but functions slightly differently.
Because their purpose is to serve the divine, their soul is not a natural one that Mirnat works with. The deities of Arvastaira have enough knowledge on soul properties to create a facsimile of one. A deity will separate a tiny piece of their own nantak in order to complete the soul of a fenvasi and bind them during a ritual known as evansurana. However, unlike a natural soul, fenvasi souls do not bond with the keirsi that surrounds it. The soul will still alter said keirsi, but as it is not a true soul created by Mirnat, it lacks the ability to claim that keirsi for itself and temporarily become as one.
Because of all the aforementioned facts, fenvasi do not get reincarnated when they die. Because of their half-divine nature, their souls contain no prior incarnations either. If a fenvasi chooses to not undergo evansurana, their death will sever the portion of their soul that contains fenvasi traits and allow them to come back as regular fenva. In addition, once deceased, their corpse will not decay either. Instead, it simply disappears after a day or two, leaving no trace behind. All the keirsi expended to create them returns to the divine parent, whereupon the deity is able to reuse that power once more. Keeping this in mind, deities are stronger when they have less fenvasi children but would thus exert less influence on the world due to their inability to use said strength freely.
Under extreme circumstances, a deity is able to manually force the return of the nantak fragment they parted with to form their child. However, this is often a last resort and rarely employed. The risk that comes up forcing the return of one's nantak from an unwilling entity will cause immense pain and exhaustion to both parties, which is further amplified by struggle. The period during and after a forceful retrieval of one's nantak is the most vulnerable a deity will ever be. As such, it is often preferable to deal the killing blow themselves instead.