The Brotherhood of the Vale is unique in history for not having been formed as a result of any artefacts or pre-existing brotherhood. Rather, the common threat of the Andals and their superiority in metalworking had led to its formation. Obscure, even by their standards, it was based on Witch Isle, the seat of House Upcliff (rumoured to be their benefactor) and was involved in raiding Andal bases and has the assassinations of at least 10 Andal Kings and countless warlords to its name. During the unification of the First Men, Robar Royce sought their help alongside that of House Upcliff. The Brotherhood agreed, on the condition that they would have greater powers if victorious and could function almost independently of the Nobility as and when required.
Robar Royce asked for a demonstration of their abilities which they agreed to. The following years saw an unprecedented string of victories among the First Men, culminating in the complete liberation of the Eastern and Northern lands of the Vale. However, the king refused to answer to their demands. An ultimatum was sent to the king on the weeks preceding the Battle of the Seven Stars. Robar responded by branding them as outlaws, to be hunted and killed on sight. Fleeing from their former allies, in a desperate gamble they turned to the Andals.
The Andals would have killed them on the spot had it not been for the actions of their new leader Ser Artys Arryn. The young knight was a reputed warrior, yet utterly inexperienced in the act of commanding armies. Histories written by Septons may claim some act of valour won him the honour of such a command. The actual truth, or as close as we can get to it suggest that due to the Brotherhood's actions, the other Andal leaders wouldn't dare to put on a crown if they wanted to keep their head (this brotherhood preferred decapitations over stabbing in assassinations). Artys was meant to be a walking target, someone, for the first men to waste their time in targeting as the experienced old men formed a strategy. If he died, well, the suit of armour could be adjusted to fit someone else.
Ser Artys for all his faults; was a pragmatist. He knew about this and also that if they won, he would be cast aside and if they lost, he would be a convenient scapegoat. So, with nothing to lose, he agreed to their proposition. Contrary to what any reader might expect, the appearance of seven stars was a coincidence, but being the pragmatist, he wasn't the type to deny an omen.
The battle happened as recorded in history, with the Andals charging and the First Men repelling them. Torgold Tollett the witless broke their ranks in the end but Robar predictably led a counterattack. Artys had 'honoured' the men who were the most vocal about setting him aside after the war with a position in the Vanguard, none except for Ser Jaime Corbray survived. Ser Artys, on the other hand, had led the rout as the stories say, yet the stories often fail to mention that the Brotherhood had methodically wiped out the scouts and outriders kept in the goat track (King Robar wasn't a fool either) allowing Ser Artys' host to rout the smaller guard kept there and win the day.
Yet men are the same, regardless of crowns and blood and no sooner had the crown settled on his head that Ser Artys denounced the Brotherhood as turncloaks and heretics and ordered that they be wiped out. Any resistance found was overwhelmed and those who survived did so by scattering and going into hiding.
The ones who went into the hills took up identities as Hill clansmen and continued to harry the Andal nobility. However, by the time of the completion of the bloody gate, their purpose was lost to them and they now live indistinguishable from the other hill clans of the region.
It must be pointed out, however, that certain traditions kept among the hill clans, such as the self-mutilation of the Burned men and the trophy collection of the Black Ears is possibly a remnant of the traditions once carried by the short-lived brotherhood.
Dornish Brotherhood (the Cult of the Ancients, functioning)
The origins of this brotherhood are lost to history, though two theories are held as distinct possibilities:
The first; that the Brotherhood was originally founded among the confederacy of kings present on the south bank of the Green blood. They were ideologically opposed to the system of unification proposed by the houses which would later be identified as the houses of the Stony Dornishmen.
The second; and more widely accepted theory is that they were the descendants of the Rhoynar warrior class who landed in Dorne in Nymeria's fleet. After the unification and pact between the Rhoynar and the pre-existing houses, they accepted Rhoynar customs such as equal primogeniture replacing their own while the Rhoynar took up the common tongue and the Faith of the Seven, setting aside their own language and religion.
The conservative warrior class, however, violently opposed this and a secret war broke out between the conservative warrior class and the ruling house of Nymeros-Martell. The older veterans had survived dragon flame in youth and all had faced death either in conquest or in the journey to preserve their way of life and they weren't ready to let it all fall apart for a pact.
Using tactics which would later be adopted against the Targaryens, they fought house Martell to a standstill. It has been suspected that they were aided at least materially by some of the former royal houses though this hasn't been proven. Driven to exhaustion and facing both rebellions and invasions, Nymeria relented and a second pact was signed.
Out of this pact, the Rhoynar were allowed to worship as they see fit, thus leading to the creation of the orphans of the Greenblood, though their language has been mostly lost to time, except in songs. The warrior class also gained protection by law and were actually employed by the Martells to maintain law and order within their kingdom. Outside of their public arm; the Shariffs which comprise a tenth of their strength, most of their actions are done in the shadows, based in the aptly named shadow-city of Sunspear.
The order evolved over time, branching out from just fighters to scholars, spies and healers. Yet overall, one ability was shared across all groups to remove those who would harm the people of Dorne, the abilities of the Assassin.
Reach Brotherhood (Order of the Green Hand, wiped out barring the Manderlys who have forgotten its purpose)From the land of Chivalry came the order of the brotherhood which was the closest to being that of the other order. Even their order, the order of the green hand, was better known as the Gardener knights. Nevertheless, they performed their duties admirably, until the civil war which saw their decimation followed by their destruction at the hands of Ser Harlan Tyrell during Aegon's conquest. Among all the versions found here, they are one of the three (arguably tied for first) best versions found.
Proto-versions existed during the age of the Sage kings who ruled a Reach beset on all sides by invaders. However, it wasn't until the arrival of the Andals and knighthood did this become a reality. Unlike other members, they scorned secrecy and proclaimed their loyalty to the Gardeners. While they were the most removed from the core principles of the order, they were still the closest the order had to a presence in the Reach. Being sworn to no petty lord but a king, the goal of maintaining order and justice rather than a search for fame and glory and serving almost entirely as a defensive or peace-keeping force rather than a standing army for the Gardeners to use in their wars of conquest, all of this made them unique amongst the order knightly orders of the time.
Despite their role as neutral mediators of disputes in the King's name, by the time of the reign of King Garth X (the greybeard), cracks had begun to show. The order at the time had several Manderlys among their members, which led House Peake to question their ability to be impartial judges to the matter of succession. The civil war which followed saw the decimation of their order and the reclamation under the Tyrells which followed reduced their powers even further. The original goals were lost by the destruction of the previous leadership till only the veneer of their duties remained. The order was in every sense of the word; hollowed out. By the time of the Conquest, this once proud order was reduced to a hegemony of second-sons, landed knights and beggared lords.
It is worth mentioning that the exiled Manderlys still claim the title of belonging to this order, but like almost all of their ties to the Gardeners of the reach, this is in name only.
Northern Brotherhood (Night's Watch, original purpose was forgotten)Despite being the oldest and longest surviving order, like all others here the ravages of time haven't been kind to it. The Night's Watch was never as storied as the songs claim, after all, there is only so much that one can expect from a band of petty lords and criminals. Yet, personally, I have to admit that, that picture is lacking. For any order to have survived and outlived almost all the houses of Westeros, they clearly are doing something right.
Yes, there was a time when a Skagosi, a Stark and an Ironborn would fight side-by-side, but the Age of Dawn has long gone. There was also a time when the song of 'Brave Danny Flint' was painfully more than a song. We remember both and yet a lot has been forgotten. A lot of the watch's failures have been noted down for scholars and lords in the South to scoff at and yet no one remains to note down their victories, for even then, the crows of the wall often don't live long enough to write about it if they bothered to.
Yet the fact remains, that their founding goals have eroded over time. Civil Wars, the attrition among the elders of the watch and time itself have made sure of that. Their failures have added over time and they must be noted; breaking of the pact with Joramun, the failure to protect Hardhome, the alienation of the Free-Folk, Brave Danny Flint, the abandonment of the New Gift and now with Mance Rayder. These failures have added up and what follows is an inevitability.
By winter, the Watch will end.
Brotherhood of the Iron Isles (The Grey King's 'children', and the Drowned men, wiped out by infighting and purpose forgotten over time respectively)The Isles never had a brotherhood proper. The closest they came to that was the formation of the Kingsmoot and yet, the Drowned Men who led it are all that remains. Their corruption of the way passed down by the Grey King, which they have appallingly titled as 'the Old Way' has led to its fall. Nothing more need be said.