The Boatmen's Society, or the Oath-Circle of the Oar, commonly called the Ferrymen by those unfamiliar with their secrets (a list that encompasses just about everybody) are one of the oldest and most powerful societies in the Underworld.
To the Hierarchy, ferrymen are a mysterious and largely inscrutable force. Charon founded their organization, and they are in this way the older brothers of Stygia, but they obviously hold current Stygian society in contempt. This obvious disgust, coupled by their obvious power (every ferryman is an engine of destruction when required) and their secrecy, means that most wraiths view them with some degree of distrust. The Ferrymen are just too powerful to not be sinister.
When he crossed over to the Underworld, Charon was recognized by the Lady of Fate as the foremost agent of transcendence in the Underworld. To this end, he gathered other restless to him, and taught them to build reed boats and pole the Sunless Sea, exploring the Underworld and guiding souls to their proper resting place.
Charon was a Mycenaean, a product of the proto-Hellenic civilization of approximately 1400 BCE. The wraiths who joined him in this time were also Hellenes, and from their breathing culture, they adapted the forms of a mystery cult for their nominal society. The Boatmen's Society became an initiatory organization, with members being chosen by vote among existing members, and a sequence of rites until a member was fully accepted by the society. The Ferrymen took as their charge the promotion of transcendence, the defeat of oblivion, and the separation of the living from the dead.
As the organization formalized and more wraiths crossed the Shroud, both Charon and the Ferrymen were forced to change and separate. Several Ferrymen explored the Far Shores, coming back as Shining Ones and telling their fellows of the paradises that awaited across the Sunless Sea. Charon, in the meantime, began forming Stygia to handle the ever increasing tide of the dead.
At the foundation of the Roman Republic, Charon began to divorce himself from the Ferrymen proper. The Ferrymen, primarily idealists, saw the Republic as a distraction from the real purpose of the Restless. More importantly, practices Charon was adopting (notably soulforging, even of Drones), were more troubling. To the Ferrymen, Charon was primus inter pares - respected, but not the God he was often seen as by later wraiths. To satisfy the Ferrymen during the foundation of the Republic, Charon went on a great Quest for Seven Signs to assure that Stygia would also be a vehicle for Transcendence.
At this time, however, the Boatmen's Society experienced its greatest change. The god Anubis, in order to protect the wraiths of Amenti, struck a bargain with the Ferrymen: in exchange for protection, the benefactor taught them secrets of the wraithly soul, culminating in the development of the Ritual of Severance. The Ritual split a Ferryman's Shadow and Psyche permanently, creating a special Spectre called a Pasiphae and freeing the Ferryman from any second-guessing or self-destruction in his own head. The ritual vastly increased the power of the Ferrymen, but it also made them the only beings in the underworld immune to internal corruption. The Severance rapidly spread through Ferryman society, even being practiced on Shining Ones. Only one Ferryman refused the ritual - Charon.
And Charon's practices became darker. Stygian society, to the Ferrymen, became a kind of mock-fetter - attaching other wraiths to the Underworld even as their other fetters crumbled to dust. Charon became more power-hungry, engaging in "frivolities" like the Lux Veritatis and the tithes he demanded of the freshly dead.
The First Great Maelstrom sundered the Ferrymen from Stygian society. When Charon declared himself Emperor of the Dead, the Ferrymen revolted and were declared Renegade. Some small number (including some severed) stayed as part of the Magisterium Veritatis, but the remainder left Stygia, vowing never to return to the Isle of Sorrows.
Since that time, the Ferrymen have subtly interfered in Stygian society. They exploit old connections (the majority are older than the Deathlords), their personal power, and their mystique to move Stygian society away from its more blatant abuses and towards its original mission. The Ferrymen have taken a more proactive approach since World War II, seeing their previous non-interference as a mark of pride and a path to Oblivion. They forced the government of Stygia to form the Covenant of the Millions following the Holocaust, establishing the Dark Kingdom of Wire.
In the days before the Sixth Great Maelstrom, Anubis called in the favor that the Ferrymen owed him for the Ritual of Severance. In obedience of his request, the Ferrymen manipulated Stygia into attacking Enoch, the city of the True Black Hand. This leads inevitably to the Maelstrom itself. During these events, they return to Stygia and Charon's reincarnated form, helping to evacuate the city as it collapses. Their fates afterwards are unknown.
The Boatmen's Society is a small organization without much active hierarchy. There are initiatory grades, but they are largely unimportant in the modern era. The core distinction is between the First (the most senior Ferrymen, an informal organization dating back to those Ferrymen who operated before the Severance), officers of the organization (who are generally among the First), and the rest of the organization. As a rule, Ferrymen treat anyone who has undergone the Ritual of Severance as an equal.
What little organization the Ferrymen have is centered around Dis, a sargasso sea of fortresses, caverns and ships in the tempest. This holds their main gathering places, the headquarters of their archival and fabrication divisions, and their small steering committee. The overwhelming majority of Ferrymen are never at Dis, focusing instead on their primary mission of traveling the underworld and helping restless achieve Transcendence.
The cornerstone of a Ferryman's power is the Severance. Once this ritual is completed, their Shadow separates from their corpus, forming a Pasiphae. If the Ferryman and Pasiphae can accept each other's existence, then the Pasiphae leaves for the Labyrinth. From that point on, the two agents operate as mirrors of each other, generally equivalently powerful.
The Severance means that a Ferryman has extra Corpus, complete immunity to Catharsis and Harrowings, and a huge amount of Pathos and Angst to draw on as needed. In exchange for this, they become a battery for their Pasiphae - sending Angst to the Pasiphae the way the Pasiphae sends Pathos to the Ferryman. In addition, the ritual has an odd effect on their corpus - Ferrymen tend to look drawn out and stretched, and it impacts their ability to cross the shroud. However, the existence of the Pasiphae is the greatest concern - every Ferryman knows they have a diligent, energetic and intelligent nemesis actively seeking to undo what they do.
In addition to these abilities, the Ferrymen have access to a variety of special Arcanoi and equipment. Not the least of which are their traditional accoutrement - their scythes, robes, boats and lanterns, each with its own distinctive abilities. They also use their skills to maintain and run the famous Midnight Express.
Finally, the Ferrymen are dangerous simply by virtue of being Ferrymen. The Boatmen's society has extremely stringent criteria; wraiths picked for this honor are already marked by Fate and have demonstrated their intelligence and resourcefulness before the Severance. Nothing that happens during their initiations makes them any less powerful, intelligent or dedicated.