Hueco Mundo is another world that exists inbetween the Soul society and Earth where the Hollows exist. the world is divided into several different sections beased on the strength of those that preside within it.

Muchos
This is the largest section of Hueco Mundo where Hollows in general exist and live closest to the barrier to the human world. they however typically dont travel to the other sections of Hueco Mundo since the ones here are typically weak and will be killed by the Arrancar or the Menos that exist in deeper sections of Hueco Mundo

Grande
30% of Hueco Mundo is Grande. it is the deeper section of the dimension and it is where the Menos Grande reside. Menos Typically stay dormant inside of this area, not usually leaving unless they wish to travel to another of the worlds which usually will only occur if a strong spiritual pressure reveals itself on earth

Las Noches
The final 20% of Hueco Mundo is known as Noches. in the deepest, darkest section of Hueco Mundo is This area, resided in by the arrancar. they are the strongest members and have their side closest to soul society so that they may travel there when they wish. also inside of Noches is a large temple where Sosuke Aizen used to control the Hollow from. Arrancar usually stay here but are free to travel to other sections of soul society as they do not fear any of the other residents of Hueco Mondo.

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Las Noches
Faction Races Allowed: Arrancar, Vaizard, Nobody, Homonculus
Description: Las Noches, The White Citadel, is located in Hueco Mundo and recognized as a central power of Hollow organization outside of the Dark Council, which tends to govern matters of the Menos Forest, leaving the plans of grandeur and conquest to the ambitious Arrancar and Hollow-like allies.

Ranks:
Numeros -: Newly indoctrinated into the ranks of Las Noches, you're given a two digit number and assigned to an Espada, following their orders with religious zeal, genuine or feigned.
Permissions Ability to perform One-Star and Two-Star level missions if posted or assigned by your Espada, once per two-weeks.

Fraccion -: A selected elite by your Espada, you're given a decimal point to your number followed by your Espada's number, giving you sway and standing over lowly Numeros.
Suggested Reiatsu: 200+
Permissions Ability to perform One-Star through Three-Star level missions if posted or assigned by your Espada.

Primero Fraccion -: The second in command and the voice of the Espada they serve when their master is unable to issue orders directly. They handle all the day to day management of the Fraccion and Numeros assigned to the Espada and discipline any who fall short of expectations. Generally, they are seen as an Espada's personal enforcer.
Suggested Reiatsu: 250+
Permissions Ability to perform One-Star through Four-Star level missions if posted or assigned by your Espada.

Espada -: Only held by Arrancar is the fabled rank of Espada. Your number is replaced by a single digit number and you are given the command over a group of Numeros who may even earn the right to call you master/mistress as your personal Fraccion units. It is said that the Espada could challenge the Gotei Thirteen and nearly brought it to ruin before. Now, your job is to regain the former glory of day's gone by as a strict leader of the agenda set forth by the King and Queen of Las Noches.
Suggested Reiatsu: 400+
Permissions Ability to perform One-Star through Four-Star level missions if posted or assigned. The ability to submit Missions to your squad's mission board, and the ability to create 1 Level 300 Technique that is unique to your Numeros for members to learn.