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Life Cycle of Hollows.
Canny Hollows: these are beings like Grand Fisher and Shrieker - normal Hollows that stay behind in Mortal World much longer than usual. Why? Either they still have unfinished business... or they were already crooked in life.
My reasoning is as follows: when a good Plus turns into a Hollow, it seeks to destroy its loved ones like any other Hollow. However, the mind of the good souls can't hold together once they're done with that. They lack killer's instinct, and might even feel guilt for breaking what was once dear to them in life. This is why they seek oblivion.
Evil souls, on the other hand, gain much more pleasure from tormenting living beings. Even after they've killed those they had a personal beef with, they stay in Mortal World, because their bestial desires won't let them leave. In a way, they still have something to do that defines them and stops them from losing their inviduality, even if it's only "kill all things":
Menos Grande Gillians: When a soul becomes Hollow, the first thing it does is destroy all things it held dear in mortal life. In most cases, having lost their purpose and consumed by their own loneliness, Hollows wander into Hueco Muendo, where they instinctively seek out other souls who died in the same way. Seeking embrace of sweet oblivion, they engage in cannibalistic orgy and fuse into Menos Grande Gillian. Fusion of souls is permanent; once eaten by a Gillian, a soul cannot be separated again.
Menos Grande Gillian's have no identity, only scattered fragments of negative memories and emotions. When such a Gillian is purified, its soul passes into Soul Society as essentially blank slate. However, due to originally being gestalt of multiple entities, such soul has much vaster Reiryoku than normal and is more likely to fragment into Zanpakuto spirit and Inner Hollow. Thus, a purified Menos Grande Gillian is very likely to become a shinigami.
Adjuchas and Vasto Lordes: As noted, Menos Grande Gillians are gestalt formed from people who died in the same way. Adjuchas happens when the amount of souls reaches a critical mass, causing a new personality to bloom from the fragmented memories. This being is effectively personification of its "cause of death". However, Adjuchas aren't stable, and must keep absorbing other Hollows who died the same way to increase in power. Should they fail to absorb other Hollows, starvation will make them regress, reducing them back to mindless beasts. Once Adjuchas has eaten enough souls, it evolves into a Vasto Lorde. However, this requires that they beat all of their challengers, letting none who manage to eat a bit of them to get away.
So, why can't Adjuchas who gets a bit of itself eaten become a Vasto Lorde? It's because losing to another Hollow means there's another being with same Aspect of Death who is more powerful. A Vasto Lorde is a Menos who has absorbed or proven itself to be stronger than all other Hollows with the same aspect of death. They are, effectively, anthropomorphic personifications of their Aspect of Death, and rule supreme over lost souls who died that way. As long as they remain undefeated, they are stable; if a Vasto Lorde does lose, it begins to regress like Adjuchas until it can destroy the Hollow that defeated it.
Arrancars: A Hollow can become an Arrancar in any point of its life, as long as it has a personality at the time. An Arrancar is a Hollow who learns to define itself by way other than its Aspect of Death, thus rising above its bestial instincts.
All Arrancars are stable. While an Arrancar can keep eating souls to increase in power, it no longer fuses new souls with the core of its person - the nommed soul only loses its reiryoku. A soul eaten by Arrancar is usually reborn as a new Hollow. In any case, eating Pluses or other Hollows is pretty much voluntary for an Arrancar - they mostly do so because there is no other food in Hueco Muendo. Because of this, Arrancars can't be held as inherently worse than humans or Shinigami.
Menos and Purification: Shinigami who don't remember their lives were Menos who got purified. As the identity of a Menos is effectively a new entity and is only born after death, Konso unfortunately wipes that all away. For purposes of Redemption, a soul which has gone through Oblivion of the Gillian phase is exempt from Hell's punishment. They become Shinigami because of huge amount of Reiryoku they accumulated as a Hollow.
Resurreccion and Segunda Etapa: Resurreccion is an Arrancars ability to retake its form as a Hollow, and consequently as the manifestation of its aspect. The power of an Arrancars resurreccion is in part directly proportional to how much the Arrancar's behaviour reflects its Aspect.
While achieving Segunda Etapa is only typical for Vasto Lordes who already rule over their Aspect, theoretically any Arrancar can achieve it through meticulous training and self realization.
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The State of the Afterlife of the British Isles
Britain’s afterlife is stable – just. Long periods of internal strife between the psychopomps of Britain, known to themselves as the mac Lir or fab Lly^r, and to others as the Fae, left ample time for Hollows to develop and grow. Now, Britain faces a deep-rooted infestation of well-hidden Menos, but the Houses of the British Isles are united in their desire to rid the islands of these threats.
Annwn – the Afterlife
The British afterlife is called Annwn, a place that reflects the most untouched corners of the British Isles in its geography. The four courts of monarchs are located in a roughly central location within a few hours journey of each other by normal walking speed. Passage between Annwn and the mortal world is achieved by passing through thick banks of mist that function as the Fae equivalent of Senkaimon. The mac Lir make use of ravens, rather than butterflies, to guide them through the misty realm between Annwn and the mortal world.
The Organisation of the Fae
The British Isles are ruled by four monarchs – one each for Ireland, Scotland, England and Wales. Each of these monarchs governs passage of souls into Annwn in the country they are tied to. They each possess three Houses under their command, each ruled by a Tiarna or Pennaeth, individuals equal to the captains of Soul Society. A Tiarna or Pennaeth is directly supported by their Dara or Ail, equivalents to the Japanese Vice-Captains. The rest of the House is made up of thirteen mionn cheangal or lw rhwymo, equivalents to seated officers, plus any other Fae who are members of the House but deemed unworthy of the distinction of the upper ranks. The specialisations of each House vary depending on the current Tiarna or Pennaeth, but it is very rare for their not to be at least one House dedicated to combat in each country at any given time. The ranks of the Houses are held for life, unless the bearer of a rank forfeits it. A House member is usually given a title and land in Annwn upon forfeiting a rank. These titles are hereditary, and their bearers and their family often become the British equivalent of the Noble Houses of Soul Society.
Every year, a randomly selected twenty members of each House are chosen to leave Annwn and go out into the mortal world and deal with Hollows as they see fit. Fae on such leave from their House are known as Fianna, and are exempt from all laws of their lords until the end of their sojourn and return to Annwn.
Each monarch also rules a small Royal House. Tiarna or Pennaeth who truly distinguish themselves are promoted to this rank to serve as the monarch’s guardians and personal champions.
Alongside the House structure exists the Aois-dŕna, a loose affiliation of bards and druids that serve as advisors to the lords of the Houses, historians, genealogists, legal specialists, and experts in Ealaín, the British form of kido.
Capabilities of the Fae
All abilities the Fae possess are mirrors of the abilities of shinigami. They can walk on air like their Japanese counterparts, and focus their training on Scileanna Laochra (Zanjutsu), Ystwythder (Hoho), Gan a Arm (Hakudo) and Ealaín (Kido).
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A Note on Language
Whilst most of the terminology for the Fae is based on Celtic languages, some, particularly the specifics and names of Ealaín, were brought to Britain from elsewhere. These use Ancient Sumerian.
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In addition, every Fae possesses a Treoir Anam, their equivalent of a zanpakutou. These possess the potential for a Rhyddhau (shikai) and a Nerthol Rhyddhau (bankai) as a zanpakutou does. The only notable difference between a Treoir Anam and a zanpakutou is that the former are quite likely to take a shape other than a sword even when sealed, spears and axes in particular being quite common, and no small number of Aois-dŕna druids possessing sickles.
Ealaín
The mystic arts of Britain function identically to kido, the spells of a given path and number function the same as their foreign counterpart.
Silakus: “Way of Destruction”
1, Taka (Push)
4, Sagir (Pale Lightning Flash): Balor! Unlid the third eye that graces your brow, and let forth the spite of clouds!
11, Nuhuš Gír (Tamed Lightning)
31, Urinti (Blood Arrow): Balor! Unlid the third eye that graces your brow, and let forth the tide of chaos!
33, Nissati (Blue Arrow): Balor! Unlid the third eye that graces your brow, and let the west crash forth!
54, Su-luh Izi (Cleansing Flames): Balor! Unlid the third eye that graces your brow, and empower me to remove impurity from your sight!
58, Imiuru (Windstorm)
63, Anurimiriu (Threefold Storm Roar)
73, Erim Nissati (Host of Blue Arrows): Balor! Unlid the third eye that graces your brow, drown out the sun, and call forth the sundering west winds!
88, Aralimiriutil (Apocalyptic Threefold Netherworld Storm)
90, Gitil (Black Ending)
96, Ašgir Mŕnu (Single Sword Immolation)
Girserusu: “Path of Shielding Arts”
1, Šaga (Captive)
4, Sigulul (Golden Chain)
8, Tamšen (Reflecting Mirror)
9, Eda (Paralysis): Balor! Lid your third eye, call up the hounds of Annwn, and grace us with the living death!
21, Urinimi (Blood Cloud)
26, Zah Bu (Hiding Light): Balor! Lid your third eye, turn your gaze away, and put the sun in their eyes!
30, Ešhu Ulul (Three-Bird Restraint)
37, Ulgu (Star Net)
39, Senbu (Shielding Light): Balor! Lid your third eye, turn your gaze to me, and guard me with thine wrath!
58, Igisar Lasar (Seeing the World, Knowing All): Balor! Lid your third eye, extend your sight forth, whisper in my ear, and let me see thine sight! The sun and moon are my eyes, the trembling ground my ear!
61, Asni E-kurbu (Six-Body Prison of Light): Balor! Lid your third eye, gift the sun unto to me, and let its halo trap them!
62, Űšukur (Hundred Spear Fence)
63, Gisigulul (Locking Golden Chain)
73, Išibalsig (Rotated Mountain Wall)
75, Aš Barzilba-an (Five Adamant Pillars)
77, Kadigir (Mouth of the Gods): Balor! Lid your third eye, and gift me with your voice! The clouds are as my mouth, and the skies are as my lungs!
81, Barů-la (Splitting Nothingness)
Geasa
Unlike shinigami, the Fae bear an additional restraint upon their power: geasa (sing. geis). These are taboos for the Fae in question that they must not commit, or lose access to spiritual power or suffer some other misfortune. Anyone can bring a geasa on a Fae, though the Fae in question must willingly accept it. Treoir Anam spirits often require their wielder to agree to a geis before granting Rhyddhau or Nerthol Rhyddhau. They also pay a part in Fae society, geasa often being included in oaths to one’s lord or in marriage vows.