Spirits

Spirits are magical existences that represent the world itself, if one were to come across a forest, a lake, or even an entire mountain, there is a high chance that those pieces of the world are governed by a spirit. Whilst a spirit "can" interfere with the world, for instance a nature spirit "could" scare off an individual about to burn their forest, but spirits seldom do so, as using their power directly is incredibly difficult and in general, spirits avoid interfering with matters of the world. Part of the reason for this, is while burning a forest might "kill" the forest in the eyes of a normal person, to the spirit the essence of the forest is still there, while the spirit may still take offense to the act, ultimately it isn't something that would threaten their existence.
This leads to the entire cause of spirits creating contracts with mortal people, as when a Iraeidin stays in their territory for a very extended period of time, it will corrupt the environment and kill the spirit. Thus why the core tenant of all contracts between mortal and spirit, is for the mortal to dedicate themselves to destroying Iraeidin.
Spirits will often roam their territory, searching for an individual that they "like" or feel a connection to, once they have found an individual they believe to be satisfactory, they reveal their true form to that person alone, before offering their contract, should the individual accept the spirit will reside in their soul and lend them power until the individual dies. However, just because they have given an individual their contract, does not mean the individual has full unrestricted access to a spirit's power, as without properly training one's body, the power would quickly kill the individual. To counteract this, the spirit puts in place a sacrifice, alongside heavily lowering the amount of magical power given to the contracted. Sacrifices exist as a process to slow down or simplify the gathering of magical power to put less strain on the user. Lastly, while the spirit may have chosen a mortal they "like" most are still very cautious in terms of becoming friendly and giving full rein of their power, even when the mortal becomes ready for it, This is due to the centuries of murder, hatred, and sadness, encompassing the history of most mortal species which lead to the creation of Iraeidin, leading to making the bond between spirit and contracted a challenge in of itself.
Spirits are a naturally rare and limited existence, as such, within the world only 149 exist.

Elements & Sub-Elements

As the Spirits represent the various environments and natural existences across the world, they are generally split into five elements these being : Fire, Water, Earth, Wind, and Light. While there are a few unique spirits who are excluded from the general five, a grand majority are lumped into one of these elements. Spirits of a particular category of elements determine the general "role" that their magic serves towards their contractor, for example, practically every fire contractor revolves around their offensive magic, whilst earth contractors are heavily defense and utility oriented.
Furthermore, in recent years it has been discovered that spirits can be classified more in-depth into sub-elements within their element. Sub-elements may differ from the imagined "role" of an element, but is ultimately still powerful despite the difference, however, it should be noted that spirits classified as a specific sub-element cannot intrude on the domain of another sub element. For example, in the earth element, there is the sub element of "Rock" and "Metal", while a rock spirit could use their powers on landmasses as a whole altering terrain and the like, they wouldn't be able to do so for an area featuring large chunks of metal (such as inside a mine).
The last major aspect of sub-elements to note, is that within the number of spirits within each rank, they are divided evenly between the total number of spirits, an example of this is how it is believed that their is 15 legendary spirits, as such, there is 3 spirits of each element, and 1 spirit of each sub-element. Nonetheless, below is a list of Elements and their corresponding sub-elements :
Fire = Fire/magma/Chemical
Water = Water/Ice/Mire
Earth = Nature/Rock/Metal
Wind = Wind/Sound/Phantom
Light = Light/Lightning/Arcane

Unique Spirits

Exceptions to the rule, Unique spirits truly epitomize the word unique, as many facets that all spirits share, they differ from. Whilst most spirits exist due to an location and environment within the world, unique spirits are brought about from "concepts" of the world, as such, it is theorized that all unique spirits are indestructible existences. While the level of power that a Unique spirit is capable of is similar to that of a legendary, they are still far more fearsome due to their unnatural magic than can defy the basic laws of the world, such as being able to stop time itself and do whatever they please during it. Despite being the quintessential spirits of the world, contractors of Unique spirits are unparalleled in their scarcity, with some not appearing for several decades at a time. Below are the nine unique spirits :

Spirit of Life

Spirit of Death

Spirit of Void

Spirit of Time

Spirit of Law

Spirit of Luck

Spirit of Darkness

Spirit of Creation

The Primordial Spirit

Spirit Ranks

Within the world of spirits their exists a hierarchy, but the hierarchy primarily comes from Humans analyzing the differences in power and sacrifices of spirits, as spirits themselves do not care much for the difference in power between one another. As mentioned previously the difference is mainly attributed to that of power, as such, there are notable abilities that differ between each rank, for example, minor spirits are not capable of influencing natural phenomena such as causing rain to come about. The Spirit Ranks are as follows : Minor, Normal, Major, Legendary, Unique.

Minor :

The lowest in terms of power, but also has the easiest access to their powers, whilst other higher ranked spirits may be able to put out far grander displays of raw power, minor spirit contractors are capable of performing magical techniques in an extremely short period of time alongside a sacrifice that is basically impossible to prevent. Additionally, due to most minor spirits being inherently friendly and not overconcerned about the personality of their contractor the bond between the spirit and contractor rise significantly faster compared to other spirits.
There are 55 total minor spirits.

Normal :

The average, the level of power displayed by normal spirits is what the common person thinks of when they think of a Spirit Contractor. However, do not think less of them because of them, even a normal spirit is capable of magical feats such as altering weather or completely changing the battlefield with their magical prowess. Due to a combination of being middling in power and not having an instant usage of magical power, some will underestimate these contractors, a mistake that is often never forgotten, as while this may seem "underwhelming" to most, the combination of a still fairly rapid usage of magic with power exceeding that of rivals, can lead to a swift and relentless onslaught of magical firepower.
There are 40 total normal spirits.

Major :

When one faces a major Spirit Contractor, most experienced and smart individuals will immediately come to two conclusions. The first is that their magical power is so hyper lethal that they must dedicate the start of the fight to discovering and potentially preventing every instance of the contractor's sacrifice (of which for major contractors are often quite noticeable and lengthy). The second, is regardless of if stopping the sacrifice is possible or not, they must subdue their opponent as quickly as possible, optimally within 18 seconds or less.
There are 30 total major spirits.

Legendary :

There are three legendary spirits per element, with 1 legendary spirit per sub element, with each of them being the apex of their sub-element. Contractors with spirits of this rank are effectively considered the equivalent of a living magical siege weapon and as such can be considered the least underestimated existence within the world.
There are 15 total legendary spirits.

Unique :

The unique spirits are quite... enigmatic, while they are theorized to be on par with legendary spirits in terms of raw power, it is hard to say with certainty due to unique spirits being extraordinarily selective, to the point where some unique contractors haven't been seen in 50 years or longer. Nonetheless, unique spirits are still considered stronger than legendary spirits for one primary reason, that being the versatility of their field of magic. Whilst most spirits are limited through their specific sub-element (such as a earth spirit with the metal sub-element only being able to use magic related to metals) unique spirits are far more free through total control of their concept (such as complete control over the concept of "time").
There are 9 total unique spirits.
(lastly below is the exact magical power of each rank)
Minor Spirit Contractor = 75%
Normal Spirit Contractor = 100%
Major Spirit Contractor = 150%
Legendary Spirit Contractor 250%
Unique Spirit Contractor 250%

Spirit Sacrifices

From a "scientific" perspective, a sacrifice can be interpreted as an action or process that allows the transfer of magical power from the spirit residing in the contractor's soul, to the contractor's body. Due to the magical load being higher for higher ranking spirits, this requires the sacrifice to be more demanding, time consuming, or even dangerous, depending on the rank of the spirit. However, for lower ranking spirits, especially minor spirits, sacrifices can be quite trivial, an example for a minor spirit could be a sacrifice to clap ones hands or click one's tongue before unleashing their magical technique.
The exact action/process of a sacrifice is decided by the spirit after the contractor has accepted the contract and they begin residing in their soul, a combination of the spirit's personality and the rank of the spirit will ultimately determine weather the sacrifice is closer to annoyance, or a danger to oneself. Lastly, it should be noted that while spirits only have one sacrifice, the exception to this rule is unique spirits, who have a "passive" and "active" sacrifice. The active sacrifice is the true sacrifice required to activate one's magical techniques, however, the passive sacrifice exists as an additional, constant, detriment to the user.

Spirit Bonding

Despite the spirit choosing a contractor, most spirits, (especially higher ranking ones) are still fairly cautious of Humans and the other races they choose. To some spirits, they are unsure if the one they picked is truly capable of making strides towards defeating the Iraeidin menace, to other spirits, they have seen so many of their contractors die, that they are unable to truly trust that the next will survive, and close themselves off from any real attachment to their contractor. However, with time and enough interactions, the contractor and spirit will bond, and with it, the spirit will begin to trust their contractor with dangerous, yet, incredible, ability. An example of this is what is known as "manifestation", which is an ability that allows a contractor to partially, (or in rare cases fully) manifest their spirit, the spirit manifests in the world using their contractor as a anchor to hold them to the material world, and unleashes their full unrestrained power. However, this devastating ability will often inflict an injury or perhaps debilitation upon the user, as such, most spirits will wait until they can trust their contractor by a large degree before allowing them knowledge and usage of this ability.
expected rate at which a character would raise their bond rank :
(note, this is a general estimate, due to the specific personality and the treatment of the contractor towards their spirit the time could take more or less time)
(note 2, time is measure in event points as there might be sessions where there are time skips, or months are spent traveling where no real bonding could occur)
Minor Spirit - 10
Normal Spirit - 15
Major Spirit - 25
Legendary - 40
Unique - 40
While most spirits will give some form of scaling reward to the magical techniques that the user wields, most spirits will give three special rewards throughout the bond ranks as follows : at bond rank 5 partial manifestation is given, at rank 9 full manifestation is given, at rank 10 a special ability is given to portray the epitome of the spirit and contractor's relationship. Partial manifestation is an ability which utilizes such immense power from the spirit that it usually includes a manifestation of the spirit itself in order for the contractor to perform the ability, although, in this current state the spirit would appear largely transparent and potentially incomplete. However, partial manifestation is not to be used carelessly, as most come with a large and often long lasting drawback for the user. Full manifestation while being a completely different ability, in terms of power and Mechanics effectively just a heavily heightened version of partial manifestation.