Espada

The Espada (十刃, (エスパーダ), Esupāda; Spanish for "Sword", Japanese for "Ten Blades") are the top-ranke Arrancar in the army of Hollow-Shinigami hybrids, are the most powerful Hollows and are the main rivals of the Gotei 13

Outline
Those among the Arrancar that excel in killing power are chosen and favored according to their strength, they are assigned a number from one to ten. Those chosen ten are called the Espada. They have their given number carved into their very flesh. Their authority dominates those numbered eleven and above. The strength of the Espada is on a completely different level then that of the Números.

At any given time, the Espada has a nominal composition of ten members, all hand-picked and ranked by Hotaru and Hana, the leaders of the Espada, the Espada as a group possess comparable strength to Soul Society's Gotei 13 Shinigami captains.

Former Espada
These are former Espada that did not become a part of the Privaron Espada because of certain circumstances

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Demotion
An Espada may be demoted from the group either when they lose strength or cease to be useful to the Primera. In most cases, they will be demoted to the Privaron Espada (十刃落ち (プリバロン・エスパーダ), puribaron esupāda; Spanish for "Deprived Sword", Japanese for "Ten Fallen Blades"), who are former Espada that have officially lost their rank and are assigned a three-digit number.

Privaron Espada
The Privaron Espada are former Espada who have officially lost their rank and are assigned a three-digit number. Being former Espada, they are far stronger than the normal Números. They reside in Tres Cifras, which translates literally from Spanish as "Three Digits". As mentioned by Dordonii, these former "naturally created" Espada were rendered obsolete after Aizen began artificially creating Arrancar to meet his own standards.

Espada exclusive Powers & Techniques

 * Caja Negación (反膜の匪 (カハ・ネガシオン), kaha negashion; Spanish for "Negation Box", Japanese for "Anti-Membrane of Negation") is a variant of the Gillian-exclusive ability Negación used to punish Arrancar subordinates by trapping them eternally in an alternate dimension. However, should the Caja Negación be used on an Espada, the Espada will eventually break out of the alternate dimension (usually within a few hours), due to their large amount of Reiatsu.
 * 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 particular, very strong Cero. To perform this potent variation of the Cero, the Arrancar first draws blood from the Cero-generating appendage using his or her Zanpakutō, then charges the Cero while mixing the blood with it. The result is a Cero with a much greater attack power and speed, as well as a change of color unique to the Espada.
 * Garganta (黒腔 （ガルガンタ）, garuganta; Spanish for "Throat", Japanese for "Black Cavity") is how 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.