Hueco Mundo is located between the World and Where Souls go (be it heaven, hell, or whatever your culture calls it), 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 use a more stable variant of the latter technique known as Garganta.
Hueco Mundo is a seemingly never-ending white desert with a black sky, with dunes like those of the Sahara. The desert is littered with what appear to be boulders and shrivelled 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. The vast desert also seems to lack any bodies of water. Which means that Grimmjow has had to steal ships from others, but this doesnt mean Hollows are new to this world they are the Souls of those who didnt pass on who lingered with problems and when a ghost becomes so filled with hate it becomes Hollow pure evil
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.
Las Noches is also located in Hueco Mundo. Formerly ruled by Baraggan Luisenbarn, King of Hueco Mundo, it is now the headquarters of a new leader who Grimmjow worked for, but when he was defeated his leader had no use for him, thought to be dead Grimmjow wandered Hueco Mundo gaining stealing souls from Gustav and turning them into Hollows he has created a formidable army that will only get bigger with death.
Menos Grande (Gillian Class)
Are the first and weakest of all Menos. In human terms they would be considered foot soldiers. There are many of them and they all look alike. They are huge monster Hollows towering at least 2-6 stories tall, easily dwarfing even the largest standard Hollow. They all have a white mask with an elongated nose. Their bodies are covered from theirs head to their feet with a black covering reminiscent of a cloak. Around their neck is a row of thin white spikes. They have large, pointed feet wrapped in white bandages. They have large white hands with long, sharp, white nails, but they are commonly hidden within their cloak, rarely revealed even in combat. They also have long tongues capable of breaking a fellow Hollow's mask.
Evolution: Though Menos Grande usually have no personality to speak of, which makes them dangerous since they have no mind of their own, one of the many Hollows that comprise it might gain control over the newly formed entity, reflected in the unique mask it wears in comparison to a normal Gillian. Only these Gillian variants have the potential to evolve into Adjuchas-class Menos. The evolution is facilitated by the continued cannibalization of fellow Hollows, even other Gillians.
Aaroniero Arruruerie is the only originally Gillian-class Menos to have achieved the rank of Espada after his Arrancarification.
Are the second classification of Menos. They are smaller than Gillians and far fewer in number. However, they're much smarter and stronger than Gillians. They're often the ones who lead Gillians into battle. Adjuchas are commonly solitary creatures rarely joining into groups.
Appearance: Adjuchas can vary radically in appearance; anywhere from bulky humanoids to the jaguar-like form of pre-Arrancar Grimmjow Jaegerjaquez.
Evolution: Adjuchas-class Menos are much smaller in population compared to the Gillian class, due to the rarity of any one Hollow being capable of overwhelming the many other Hollows that comprise the body of a Gillian. Furthermore, the Adjuchas must continue to devour Hollows, or its mind will be absorbed by the other Hollows comprising its form and it will revert back into a Gillian. The subsequent lack of an individual will also preclude the reverted Gillian from becoming an Adjuchas again.
Are the third classification of Menos and the highest level of evolution. They are extremely small Hollows, roughly the same size as humans and are extremely rare in number. It is said that you could count their entire number within Hueco Mundo with only the fingers of your hand. However Grimmjow has been guiding the Gillians through the Evolutinary chain, but they still remain rare.
Apart from the need to cannibalize even more Hollows, any special requirements for the creation of a Vasto Lorde are unknown.
An Arrancar is a Hollow that has removed its mask, thus gaining special-powers like that of Elite soldiers from other realms.