Kaisou

A Kaisou is a collection of soul fragments that merged together in the Dangai to form one being. Despite having existed for a very long time, the Shinigami only realized their presence when the Kyuushoku, an organization of Kaisou, began to wage war on the Soul Society. While they are a remarkably powerful race overall, they are also prone to being mentally and physically unstable due to the fact that they have several pieces of several souls all residing in one body.

Creation
When a person is caught in the Dangai, they get sent back in time by rapid, 100-year intervals. The stress this places on the soul causes it to crack, and eventually shatter. The pieces are swept away by different time currents, scattering them throughout Earth's history. In an attempt to stabilize themselves and make themselves "whole" again, these fragments will attempt to merge with other pieces of different souls. If enough pieces fuse together, a Kaisou is created. While the exact number of soul shards needed to create a Kaisou is unknown, it has been noted that the fewer the fragments, the more stable the Kaisou.

There are a small number of instances where pieces of a Shinigami's soul have merged together with those of a Hollow. This results in a hollowfied Kaisou, which essentially grants that individual the powers of a Vizard. While the power gained from such a combination is phenomenal, it tends to have a largely negative impact overall due to the way that the Shinigami soul(s) and Hollow soul(s) will be constantly at war with each other. As such, Vizard Kaisou are actually considered to be far less valuable than the typical Kaisou, as the latter is less likely to self-destruct.

Appearance
While no two people look alike, there are a few traits shared between all the Kaisou. Their eye and hair colour will always match, and they are not restricted to the natural browns, blondes, reds, and so forth. Their eyes also glow in the dark, which makes them easy to pick out during the night or in rooms with dim lighting. The reason for their glowing eyes is the presence of the tapetum lucidum, which allows them to see better in low light. This helps them to better navigate and make use of the Dangai.

If a Kaisou is a member of the Kyuushoku, they will have a broken chain tattooed somewhere on their body. The most common location is about halfway up the forearm, although there are those who have chosen to have it done elsewhere. Some are also more creative or flashy than others.

Behaviour
Due to the way that they were created, the Kaisou are not particularly stable in terms of their mentality. The soul fragments inside them are essentially at war with each other, fighting for total dominance over the body, which tends to lead to very erratic and unpredictable behaviour. Each fragment is essentially a piece of it's own person, so there could be countless people housed inside one physical shell. Often times, the largest fragment becomes the most dominant personality. In some cases, however, the various souls within the Kaisou have a mutual agreement to work together, although this is remarkably rare.

Powers
Unlike many of the other souls, the Kaisou only have a set range of powers. While they can choose any which one out of the five available, the stability of their fragmented soul often dictates which powers they cannot take, often leaving them with only one or two choices. Another thing to note is that despite being made up of Shinigami, Kaisou can't actually wield a Zanpakuto, because their soul isn't "whole." They can, however, use Kido and Shunpo, although very few use the former because they cannot use it well enough to cause the amount of damage they need.
 * "Shadow" Manipulation: The name of this technique is not particularly accurate, given that they don't manipulate shadows, but rather a kind of dark energy.  What they do with this dark energy depends solely on the individual and how much power they have.  Some can will it to take the form of an animal, or an army, while others could use it to coat their body in some kind of armour.  They can will it to be soft or hard, sharp or dull, gaseous or solid; just about anything is possible with shadow manipulation.  Most of the Kaisou are able to do this to some degree, although only a percentage of them are actually good enough at this to use it in combat.
 * Shape-shifting: Since the Kaisou's body is actually nothing more than a reflection of what the souls inside have subconsciously willed it to be, it's possible for them to change their outward appearance into something else.  It should be noted that this technique is remarkably difficult, and requires a lot of willpower and "teamwork" among the soul fragments that make up the Kaisou itself.  All the fragments have to agree on the transformation, and it's because of this that very few of the Kaisou are able to shape-shift.  Those that have the stability to do change their form have very few restrictions otherwise, and can change into living things, inanimate objects, and even other people depending on how powerful they are.  However, if they do turn into others can always be identified by their eyes remaining the same colour as their hair (so if the person had orange hair, they'd have orange eyes too), as well as the fact that their eyes will still glow in the dark.
 * Gravity Dome: This power is the only one that makes use of physical objects, and without them, it simply won't work.  Before or during the battle, the user must make a perimeter or sorts by placing seven stakes on the ground.  The shape they make doesn't matter.  These stakes create a domed barrier, and allow the user to manipulate the gravity within the enclosed area.  While placing the stakes is fairly easy, learning to control gravity and being able to keep it up for long periods of time is another story entirely.  More Kaisou choose shape-shifting over this, but those who persevere are rewarded with being near-invincible once they erect the dome.
 * Sound Manipulation: Like shadow manipulation, sound manipulation is much more diverse and easier in comparison to shape-shifting or the gravity dome.  It allows the user to use sound waves (often ones that they've projected) for offensive and defensive purposes.  It is arguably the easiest power the Kaisou have access to, although it takes a true master to bring out it's full potential.
 * Breath Weapon: Kaisou are actually capable of things like breathing fire or toxic gas, but they have to modify their body somewhat in order to do so.  Using similar techniques as observed in shape-shifting (changing one's appearance through thought), it's possible for them to go as far as generating a new organ.  This is done by "killing" a few of the soul fragments and using their energy to create said organ, which will generate and hold the chemicals and gasses needed to spew fireballs or whatever else.  The more fragments given up, the larger and more powerful the organ will be.  In other words, the more unstable the Kaisou is, the better-suited they are for this particular power.

Abilities

 * Access to the Dangai: Kaisou are able to open gates to the Dangai.  However, unlike the Shinigami who can only access one path, Kaisou can force the gate to open up other paths or "pockets" where there is a gap between the currents of time.  Since neither the Shinigami or the Kototsu can access these places, it makes it a perfect safe haven for the Kaisou, and has allowed them to go undetected for as long as they have.  It is also how they infiltrated the Soul Society in the first place.


 * Kaikyō Kotei (界境固定, Realm Boundary Fixation): The Kaisou are more than capable of stopping the flow of the Kōryū, which allows them to remain within the Dangai for an indefinite period of time.  This is done by filling several Seishin no Kabushiki with reiatsu, and then placing them in the Kōryū itself.  It's not entirely unlike the method the Shinigami use to stop the flow, but it is far more efficient, given the amount of effort the Kaisou have put into perfecting this technique.

The Kyuushoku Clans
The nine original Kaisou clans, these were formed over thousands of years as more and more Kaisou sought each other out for protection. They make up the majority of the Kaisou population, and are also the ones who formed the Kyuushoku.

While the clans use different methods to determine the leader of the clan, one thing remains the same; once a leader has been chosen, they must change their name to that of the clan's. For instance, the leader of Clan Grey is named Grey, or the leader of Clan Azul is named Azul, and so forth.

Clan Kālō
Located in Southern Asia, Clan Kālō is considered to be the most powerful in terms of physical strength. They have the second-largest population, topped only by Clan Lila. Aggressive and fuelled by hatred toward the Gotei, they were killing Shinigami long before the start of the war, and have been the largest driving force behind the Kyuushoku. Their current leader is considered to be the unnofficial leader of the organization, as he was the one who united the clans to take revenge on the Soul Society.

Clan Grey
Clan Grey is one of the smaller, less-powerful groups, and has made their home in Western Europe. They used to be one of the biggest powerhouses, but a fight with Clan Glas over boundaries largely diminished their numbers and the amount of land they "owned." They were among the first to join the Kyuushoku in an attempt to "prove their worth" to the rest of the clans, and try to restore themselves to their former glory. The current leader is thought to be the most competent Grey in a number of years, and he has been an essential part to Clan Grey's slow but steady ascent to power.

Clan Baltas
Clan Baltas is the most peaceful clan, but arguably the most dangerous as well. It's rumoured that they outmach Clan Kālō in strength, although nothing has been confirmed. They reside in the northern regions of Europe, and have mostly kept to themselves up until now. As such, very little is known about them.

Clan Akai
Smaller than Clan Grey but far more powerful, Clan Akai can be found in Japan and both North and South Korea. Originally part of Clan Chéng, they separated sometime in the 1500s after the then-leader made some decisions that they disagreed with. Since then, there has been a remarkable amount of tension between the two clans, and boundary disputes are all-too common. Even the formation of the Kyuushoku hasn't soothed old wounds, and they continue to quarrel with Clan Chéng.

Clan Chéng
Covering the remainder of East Asia, Clan Chéng is small in terms of population, but is still one of the biggest powerhouses in the Kyuushoku. They've been fighting with Clan Akai since the separation, and while the war against the Gotei has stopped any physical conflict, the grudges they bear toward the other clan are crystal clear.

Clan Kítrinos
Clan  Kítrinos is considered to be the oldest of all the clans, and is a force to be reckoned with. They enjoy a good relationship with Clan Azul, with whom they share the southern regions of Europe. Prizing tradition, they've changed very little since 8th Century BC in terms of their lifestyle. That being said, they have kept up with the times, but have simply chosen not to employ all the newest technology.

Clan Azul
Like Clan Baltas, little is known about Clan Azul. They opted to stay out of the spotlight for the most part, with the only defining moment in their history being the day they decided to form a good relationship with Clan  Kítrinos.

Clan Glas
The only one of the nine clans to focus on advancing technology, Clan Glas is full of inventors and creators. They were the ones who developed the armour, weapons, and overall tools that the Kyuushoku uses today, and because of that, are an essential part in the war. They maintain a very friendly relationship with Clan Lila. They aren't overly friendly with Clan Grey, however, given that they overtook most of said clan's territory in Western Europe.

Clan Lila
The newest but largest Clan, Clan Lila is spread throughout all of North, South, and Central America. Their current leader was originally part of Clan Grey, but overcrowding in the eastern part of the world caused her to leave and create a clan of her own elsewhere. Clan Lila is now the second most-powerful clan in terms pf physical power, and could probably give Clan  Kālō a run for it's money. They are considered to be the "modern" clan, as their lifestyle has changed to resemble that of the average person's in the 21st Century.