Intro:
Un Eternidad de Hueco (A Hollow's Eternity) is in Arrancar and Hollow Mythology the Nirvana Arrancar and Hollow version of the afterlife, and is where Honorable Hollows and Arrancars go to after they die.
Overview:
Un Eternidad de Hueco is the Hollow and Arrancar version of the afterlife where all Hollows and Arrancars are believed to go after death. However for the former, only Hollows that were born Hollows are allowed to pass on into this plane of the afterlife, Hollows that were once Human DO NOT pass on to Un Eternidad de Hueco after their death, for they get purified by a Shinigami's Zanpakuto, which allows them to pass on into the Soul Society instead. And as for Arrancars, Arrancars that were once Hollows and were born Hollows before they evolved into Arrancars, as well as Arrancars that were born Arrancars. Along with Arrancars that were once Human but became a Hollow after having their souls corrupted by the Hollow God Hueco (and to Menos-class Hollow if possible) then into a Arrancar, also get to pass on to Un Eternidad de Hueco after their death. Because once they have already evolved into an Arrancar (or if they had evolved from a regular Hollow into a Menos-class Hollow before evolving into an Arrancar), the end result is that their souls were already FAR TOO corrupted by their evolution into an Arrancar (or by their evolution into a Menos-class Hollow before becoming and evolving into an Arrancar), then even a Soul Reapers Zanpakuto CANNOT purify them. Because upon evolving from a Hollow into an Arrancar, or even evolving from a regular Hollow into a Menos-class Hollow, then into an Arrancar, their humanity would have already COMPLETELY disappeared from them, and they would have ALREADY become a full-fledged, pure-blooded, full-blooded Hollow, just like the Hollows that were born Hollows, and the Arrancars that were once Hollows, and were born Hollows before they evolved into an Arrancar, and the Arrancars that were born Arrancars.
Un Eternidad de Hueco is the Hollow/Arrancar version of Heaven. It is the place where honorable Arrancars and Hollows who were not truly unrepentantly complete monster evil go after death. After death the deceased Arrancar/Hollow will be greeted by the Los Doce Celestiales (The Twelve Celestials) twelve powerful Arrancar deities who will then judge the deceased Arrancar on all of the things that he/she had done in life, just to see if they are worthy of entering into Un Eternidad de Hueco. And if the Arrancar or Hollow has shown honor in any way, shape or firm, (which also includes dying with honor) no matter on how evil they were in their life previous life (as long as they were not unrepentantly complete monster evil). And if the Los Doce Celestiales approve of what the Arrancar/Hollow has done in his/her previous life from their birth to the day they die, then the twelve, Arrancar Gods will permit the deceased Arrancar/Hollow to enter into Un Eternidad de Hueco, where they will be greeted by Abogado de Nirvana (Nirvana's Advocate) another powerful Arrancar deity who will grant them their new lives in Un Eternidad de Hueco.
However if the Arrancar/Hollow had shown complete dishonor, cowardice, and absolute evil in their life, and died in complete disgrace. Than the Los Doce Celestiales will reject that Arrancar/Hollow, and banish him/her from Un Eternidad de Hueco PERMANENTLY, and send the dishonorable Hollow/Arrancar to another dimension, into another plane of existence without the presence of Hueco, which can be either Hell, or another plane of existence that is basically the Hollow/Arrancar equivalent of Hell, Eterno Olvido Hueco (Eternal Hollow Oblivion). Where they will be condemned to eternal damnation and an eternity of endless torture for their sins during their time alive, just like the Christian Human and Human-turned Hollow version of Hell.