Hueco Mundo (虚圏, ウェコムンド, Weko Mundo; Spanish for "Hollow World", Japanese for "Hollow Sphere") is the dimension where Hollows and Arrancar usually reside. Since both Sosuke Aizen and Akujin's departure from the dimension, it also the home of a mass group of Vizard called the Kinenbuke.
Hueco Mundo is located between the Human world and Soul Society, though it is separate from the bordering dimension known as Dangai. It can only be accessed by tearing the dimensional fabric separating the worlds, allowing Hollows to travel to any other dimension at will. Depending on the Hollow, the effect of this process varies. Regular Hollows usually appear from a small black vortex or form out of smoke, while Menos-class Hollows quite literally tear the sky apart to emerge. Arrancar and Vizard use a more stable variant of the latter technique known as Garganta.
The landscape of Hueco Mundo is a seemingly never-ending white desert, with dunes like those of the Sahara. The desert is littered with what appear to be boulders and shriveled trees, but these are actually a form of quartz. In Hueco Mundo, the moon is on the opposite lunar phase of that in the Human world. Like in Soul Society, the atmosphere is filled with Reiryoku in the form of Reishi, but it is more concentrated, thus allowing smaller Hollows to gain nourishment despite the lack of Human souls. This reishi also allows Vizard to sustain their powers longer.
Like that in real world deserts, active life in Hueco Mundo appears to be few and far between. Hollows generally seem to dwell below the surface or in other areas until something provokes them to leave their shelter, like the intrusion of a foreign entity. According to the Shinigami, all Hollows dwell in Hueco Mundo regardless of their shape, size, or power. Hollows can also drag other spirits into Hueco Mundo.
Las Noches is also located in Hueco Mundo. Formerly ruled by Barragan Luisenbarn, former King of Hueco Mundo, and it was also once headquarters of Aizen and his army of Arrancar. It is now the home and base-of-operations for the Kinenbuke.
There also exists a vast forest underneath Hueco Mundo where Menos-class Hollows reside. The trees in this forest extend to the surface and dot the landscape above ground. The Adjuchas-class Menos which reside in this forest were formerly under the control of Aizen.
Accessing Hueco Mundo
Shinigami from Soul Society enter and leave Hueco Mundo by unlocking the Senkaimon using a special Kido spell instead of a Hell Butterfly. Vizard, Hollows, and Arrancar on the other hand, enter Hueco Mundo through the technique called Garganta. Spirits can be born in Hueco Mundo as much as in the living world or Soul Society, though they are called Hollows, with the Shinigami-Hollow hybrids called Vizards acting to balance the souls.
King of Hueco Mundo
Hueco Mundo is ruled by a king. Though, since Barragan's death he is merely a figurehead, his existence is undeniable. He commonly leaves the running of Hueco Mundo to the Kinenbuke and Espada. He lives in a palace deep within the dunes of Hueco Mundo, protected by Hollows and Arrancar of every rank.
Main article: Kinenbuke The Kinenbuke (lit; Soul Army) are a sect of Vizard who utilize hollow powers. The term "Kinenbuke" originally referred to a species of beings native to Hoka Seken. These Vizard had once been members of the Order of the Vizard, a monastic group dedicated to peace through the use of their hollow powers. Then for reasons unknown, they left the Order and became their own group.
The Kinenbuke are similar to the Espada in one way. Like the Espada the Kinenbuke follow a certain aspect. The Espada each follow an Aspect of Death. On the other hand, the Kinenbuke each have an Aspect of Pain such are the following; Attachment, Loss, Love, Betrayal, Abandonment, Idealism, Anger, Desire, Anxiety, Rejection, Resentment, and Guilt.
Later on, after Bleach Otrosendero - Yūyake no Sōzōshin, vizard have become so numerous throughout the world that Ryan decided to find another way to organize vizard without the ancient Order of the Vizards, because no one could trust the Order and, by similarities, any vizard due to the Suneku incident. Ryan then decided to base the Vizard in Hueco Mundo. They took over the abandoned Las Noches and founded eleven divisions that functioned similar to the Gotei 13. He calls their new job to protect the human world instead of Soul Society. While the Espada are not considered part of the Kinenbuke, they work together with them and reside within a underground chamber in Las Noches that resemble the Menos Forest in the outside Hueco Mundo.
Main article: Espada The Espada are given free control over the lower less complete arrancar in Hueco Mundo and may choose as many of them as they like to become their Fracción. Similar to the power gap between captains and lieutenants in the Gotei 13, the strength of the Espada far surpasses that of the average Fracción.
Their immense amount of power and leadership status among the arrancar also grants each Espada the authority to exert command over most other arrancar within their territory. Unlike their Gotei 13 counterparts, there seems to be a sort of hierarchy among the Espada; a higher-ranked member apparently can give orders to lower-ranked members. Various missions usually involve one or more Espada operating together, with the most senior member exercising command over the others; typical, non-combat missions, such as searching for the Vasto Lorde and gathering pre-invasion data in the human world, include a senior Espada and a lower-ranked one being sent out in pairs. While the lower-ranked members usually have little restraint in taking their own initiative (thus exceeding their orders), higher-ranked members usually stick to the mission, and are often the first to bail the others out of trouble, because, they wish to "prevent the Espada from losing one of their own".