Race: Hollows

Hollows

Brief Description:
Hollow (虚 (ホロウ), horō) are a race of creatures which are born from Human souls who, for various reasons, do not cross over to Soul Society after their death and stay in the Human World for too long. They are corrupt spirits with supernatural powers that devour the souls of both living and deceased Humans. Hollows are the principal antagonists at the beginning of the story and remain a primary threat beyond that point. Hollows settle in Hueco Mundo, but can cross over to the Human World and Soul Society.

Read more here:
Hollow Wiki

Their World:
Hueco Mundo (虚圏 (ウェコムンド), Wekomundo; Spanish for "Hollow World", Japanese for "Hollow Sphere") is the dimension where Hollows and Arrancar usually reside. They are currently assaulting Seretei, but are cut off from the Living World due to The Seal set up by the Superhuman Council.
Learn more:
Hueco Mundo Wiki

Within Their World
Las Noches (虚夜宮, Rasu Nōchesu; Spanish for "The Nights", Japanese for "Hollow Night Palace") is the fortress located in Hueco Mundo and base of operations of the Espada. They reside there with their army of Arrancar complete with medical facilities and detention wings. They reside there with their army of Arrancar complete with medical facilities and detention wings.

Look here to learn more:
Las Noches Wiki

The Forest of Menos (メノスの森, menosu no mori) is an area below the surface of Hueco Mundo's desert where the average Hollow hide from the Arrancar. Gillian- and Adjuchas-class Menos dwell here, along with possibly Vasto Lorde-class Menos. The forest is filled with large trees made of a silvery quartz substance. These trees extend to the surface and dot the landscape of the desert in Hueco Mundo. The Adjuchas in charge of the Forest of Menos are under the command of the Patron Saint of Protection.

Find out more:
Forest of Menos Wiki

The Hierarchy
Hollows are generally disorganized and unruly, usually only forming under a leader if they are forced to. In fact, their very nature is to look out for themselves, and the strongest attack each other in order to gain power. It is well known that the most organized Hollows can get is a small group, usually among the Adjuchas class and higher. So in order to create a hierarchy and structure within the ranks, the Espada have adopted a theocratic form of government, with the Primera Espada taking position as Godking and the rest of them taking the position as Patron Saints. With this system, the Hollows follow the belief system and will receive orders from their leaders. The Patron Saints and a basic description of their jobs are listed here:

Godking: This title is always held by the Primera Espada and is the most important position in the religious hierarchy. The Godking is meant to be a spiritual figure for all Hollows to worship, and is generally agreed to be the strongest Arrancar that exists at the time. He or she is the Commander in Chief of the Hollow army, as well as the leader of the Espada.

Stealth: The Espada of Stealth is the leader of all covert operations, and the Patron Saint of secrecy. He or she uses furtiveness and trickery to achieve ends meet, and is sent for assassination or retrieval missions.

War: This Patron Saint of War is the top General in the Espada army and a strategical genius. His or her job is to make plans for battle and, with the help of the Godking, direct forces and the Patron Saint of Leadership during skirmishes.

Leadership: The Leadership Espada’s job is to manage the forces during a battle and, generally speaking, battle the top enemy warrior. During the battle, he or she has communications with the War Espada and Godking in order to achieve mission objectives.

Knowledge: The Espada in charge of Knowledge is there to create new technologies, manage surveillance, and assist the Patron Saint of Travel in managing the Dangai. He or she also is in charge of finding enemy weaknesses, as well as assist the Healing Saint in finding biological exploits. There main objective is to find a way to reach the living world.

Healing: The Patron Saint of Healing controls the infirmary of Hueco Mundo and the field medics sent out on missions. He or she can also manage and administer medicinal drugs to Hollows, and, with the assistance of the Knowledge Espada, explore the biological weak points of foes.

Combat: The Combat Espada is in charge of training other Hollows in the way of their sword. He or she can be sent as an alternative Commander in missions instead of the Patron Saint of Leadership, but generally aids the Protection Espada in defending Hueco Mundo and Las Noches.

Evolution and Exequias: This Espada’s job is to search for Gillian Adjuchas class Hollows who are strong enough to take off their mask and become Arrancar. No Hollow can evolve without meeting with this Patron Saint. He or she also doubles as the Exequias and punishes Hollows, generally with the Caja Negación, a device that only he or she can distribute.

Protection: The Patron Saint of Protection defends Hueco Mundo from enemy attacks, most potently Las Noches. He or she controls the corridors of the fortress, and designs the traps with the help of the Patron Saint of Knowledge. Because Hueco Mundo is cut off from the Seretei and Gensei, the Protection Espada’s current mission is to defend the Dangai.

Travel: The Travel Espada is meant to manage the Dangai and organize large-scale invasions. At the moment, this Patron Saint is observing the Dangai with the help of the Knowledge Espada.

Their Powers
-Cero (虚閃 (セロ), sero; Spanish for "Zero", Japanese for "Hollow Flash", 'Doom Blast' in the Viz translation) is a highly powerful and relatively swift energy blast that can be fired from various body parts, such as mouth, tongue, hand or fingers. Only Menos, Arrancar, and Visored have so far been shown to use Cero attacks, and it appears that stronger entities can use it more efficiently and forcefully. Cero also requires some time to be used effectively, so the enemy must be either not moving or busy with something else, or too badly wounded to even simply dodge it, with the exception of a few Cero techniques and users. Additionally, there exists quite a number of variations of Cero, which are usually unique to the Arrancar using it.
-Garganta (黒腔 (ガルガンタ), garuganta; Spanish for "Throat", Japanese for "Black Cavity") is how Hollows and Arrancar move to and from Hueco Mundo. It literally tears open the dimensional fabric separating the worlds, revealing a tunnel of whirling, torrential energy that must be focused and solidified to create a discernible pathway.
-High-Speed Regeneration is a supplementary ability possessed by many Hollows, as well as some Shinigami that have undergone Hollowfication. It allows them to recover from any damage that they sustain by causing them to heal at an increased rate, closing the wounds on their body and causing any limbs that they have lost to grow back. Despite the many benefits that it provides, most Hollows choose to abandon the ability to regenerate when they become Arrancar, in exchange for a greater amount of power. Regardless, when Arrancar use their Resurrección, their wounds heal away instantly
-Zanpakutō: The Arrancar seal the nuclei of their abilities within the form of a sword. It is entirely different from what Shinigami use. When they release their Zanpakutōs' seals they unleash their true power and their true form. The combat efficiency of an Arrancar with a released Zanpakutō is increased several times over. Therefore, an Arrancar's release of their Zanpakutō is merely a temporary restoration to their former selves, granting them any powers their Hollow form possesses. The act of release is called Resurrección (Spanish for "Resurrection"), which is activated with a command phrase followed by the name of the sword. An Espada's Resurrección is nothing like that of a normal Arrancar.
-Bala (虚弾 (バラ), bara; Spanish for "Bullet", Japanese for "Hollow Bullet") is an Arrancar-exclusive technique that hardens the user's Reiatsu and releases it like a swift blast of spiritual energy. While the technique is not nearly as powerful as a Cero, its speed is 20 times faster and allows for it to be fired in quick repetition. In addition, it possesses more concussive force than a Cero does. A Bala's speed and power varies with the Arrancar using it
-Hierro (鋼皮 (イエロ), iero; Spanish for "Iron", Japanese for "Steel Skin") is an Arrancar technique in which the user's reiryoku condenses, creating a steel-hard skin strong enough to block a Zanpakutō. This also allows them to fight some of the sword-wielding Shinigami barehanded. The strength of Arrancars' Hierro depends on their spiritual power.
-Sonído (響転 (ソニード), Sonīdo; Spanish for "Sound", Japanese for "Sound Ceremony") is a technique used by Arrancar which allows them to move at extreme speeds, making it highly similar to the Hohō (or Flash Steps as they are called in the English Dub) that Shinigami use. While somewhat similar to Hohō, Sonído appears to be more instinctual than Hohō, requiring no previous knowledge to perform. It also makes a different sound; rather than the swishing or sometimes silent Flash Step of the Shinigami, Sonído makes a booming or static sound.
-Gran Rey Cero (王虚の閃光 (グラン・レイ・セロ), guran rei sero; Spanish for "Grand King Zero", Japanese for "Royal Hollow Flash") is a Cero that only Espada are capable of using. It is performed by mixing an Espada's own blood with the Cero. The result is a Cero with a much greater attack power and speed as well as a change of color unique to the Espada. It is also many times more powerful than a normal Cero, capable of disturbing the fabric of space, it also seems to spin like a razor upon release as it compresses and decompresses due to the sheer speed and raw power. It is a drastically potent Cero, used only by the Espada themselves.