the Arrancar.

Synopsis;

Openly defying Seireitei to obtain an artifact of Kisuke Urahara's prior research- the Houkyuko- Aizen Sosuke forsook Soul Society to carry on his ambitions from Hueco Mundo without interference; declaring his ambition of ascension.

With the Houkyuko - a device to give Hollow Shinigami capabilities, in turn increasing their potential like Hollowfication did for the Vaizard - Aizen set out to create an army of Arrancar. And who better to use against the Shinigami than their principle enemies; the Hollow?

Note; some Arrancar have been known to evolve on their own, without necessitating the Houkyuko.

the Arrancar;

Arrancar are Hollow that have had their masks removed and have gained the powers of a Shinigami. They gain much more humanoid appearance in the process, as well as strength and intelligence beyond the level of a simple Hollow; accompanied by a strong sense of individuality. Like Shinigami, Arrancar possess zanpakutou.

Unlike Shinigami, whose zanpakutou are manifestations of their owner's soul, Arrancar's zanpakutou are manifestations of their former hollow selves. With occasional exceptions, an Arrancar's zanpakutou resembles a regular sword in its unsealed state. When released through a process called Resurrección, part of their original Hollow form is restored, changing their appearance, giving them greater strength, and granting them access to their original abilities.

Segunda;

Segunda is a second transformation which exceeds an Arrancar's Resurreción, drastically increasing their power and abilities in unique ways. Ulquiorra Schiffer claimed he was the only Espada who had attained Segunda to his knowledge.

Note; Segunda is reserved for a very few Espada who have proven their capabilities and earned recommendation from the Captain Commander.

the Deprived Blades;

Originally, the Privaron Espada were true Espada; but because Espada are ranked in accordance of power, they were supplanted by the stronger Arrancar Sosuke Aizen was able to create using Urahara Kisuke's Hougyoku device.

Privaron Espada are charged with defending Las Noches with their very lives, and to be defeated means to die in their obligation. As the strongest Arrancar beneath the true Espada, set apart from the Numeros, they retain some of their former authority and are admired by those ranking below them.

Arrancar of the Privaron Espada are the only ones aside from the Espada with a tattoo of their respective number; a triple digit number signifying their fall from prestige.

Example: Each Espada is given a tattoo in order of their power, but when an Espada is replaced and cannot retain a position as one of the ten most powerful, their tattoo is altered accordingly; if a Privaron Espada was previously the 1st Espada, their tattoo would become "101."

Arrancar Black Ops;

In order to maintain total efficiency and order within Las Noches, the Exequias were formed to parallel the obligations of Privaron Espada as well as undertake missions of internal affairs. The Black Ops. group works directly under the the Commanders and answer to no-one else to ensure the purity of their orders.

Exequias are charged with defending Las Noches from intruders and insurgents as well as disposing of rebel Arrancar. Because they aren't as powerful as the Espada - or even the Privaron Espada - they hunt their prey in a pack and are generally seen in groups.

Numeros;

Numeros are just that; the most numerous form of Arrancar. Considered the "soldier" class, their ranks are insubstantial as they don't hold any pivotal positions. Numeros are appointed tasks in accordance with what needs to be done at any given time, generally forming small squads with a higher ranking expedition leader assuming command until the mission is complete or they are relieved of their duty.