omake because i am paranoid
(Jchrispole gave me permission to adapt his 40k stuff so long as I gave him credit, so heres an adaptation of his undergangs of brine, expanded and updated to fit in the galaxies of darkness.)
Undergangs of the Brine Galaxia
No place in the Imperium is free of crime, not even Terra. While the inviolate laws of terra and the creed are followed for the most part, the Lex Imperialis is a byzantine thing with many contridictary laws that not even the most dedicated of Arbities can enforce.
Within the Imperium, money is power and crime does pay. Certain criminal organizations have enough graft to gain status equal to a noble house, most notable of which are the gang houses of Necromunda; each of which controls a single galaxy in the necromunda super-galaxia. They are not the only such organization, and a few such houses come from the Brine Galaxia. Like the necromunda houses, they control certain aspects of society that makes it impossible to dislodge them and have influenced the gangs of countless criminals throughout the Garden, though to a lesser extent due to there focus upon ocean worlds.
House Akwedok
In the underwater hives and cities, water filtration is a must. Even back on Old Earth, the rhyme of the ancient mariner instilled this with the line, "Water, water everywhere and not a drop to drink". House Akwedok is without equal in the business of water filtration, and most of the pumps that filter water into a drinkable form in the Imperium are controlled by gangs that descend or serve House Akwedok.
There power and influence is such that they can shut off entire networks of pumps, causing instant floods or droughts to demonstrate there power. They typically use this as a sword of Damocles when negotiating with other gangs or the administratium at large. In spite of this, they still war with other gangs over highly trivial matters, and against the foes of the Imperium when necessary,
They are one of the most necessary and demanding of the undergang houses, and all who claim alligence to this house wear full body diving harnesses even in the middle of a desert.
The Presidum (House Politburo)
The strongest Undergangs in the Imperium are those who get most of their profits by selling to the Astra Millitarum and the Imperial War Machine. The Hive World of Jorus underwent a sudden political restructuring in M38, becoming ruled by a unified criminal syndicate. The victorious house then put the suddenly unemployed masses to work creating hydrophobic diving suits.
Normally the imperial guard would destroy this hyped up gang, but the leadership of this gang cut a deal with the lords of the Brine Galaxia. They would create the suits used by the Brine regiments known as the Drowned Men, and in exchange they would be given control of the hive world and enough resources to fund there own army.
This revolution was managed by a group of militant radicals known as the Presidum who went through much the same downward spiral as the Imperium as a whole. At first, they fought for ideals of collectivism and protecting the weak, but through constant schisims and leadership deaths in constant bloody coups left only power hungry tyrants. These tyrants, after making the deal with the Brine commanders, would co-opt certain crusades with the drowned men regiments under the guise of suit upkeep and establish themselves as the dominant powers of the reclaimed worlds much like how regiments of the imperial guard do.
The Presidum's relations with the Astra Militarum enables them to give there gangs power armor and heavy stubbers. While no real match for a Peringa or a Space Marine, they are the closest in strength that the average ganger will ever encounter. This is in spite of the average Presidum ganger being both undertrained and heavily traumatized, but the tech they field has no march among the average criminal underworld.
House Oldenberg
One of the few mainline sects of the Imperial Creed in the water world underhives, also known as the Ice Temple of the Diad, they belive that more psykers and esoterics should be sanctioned so as to protect them from the darkness and false teachings that threaten them.
In charge of the ice harvesting trade in whatever world they come from (this creed having long since spread from Brine), taking valuable ice to volcanic regions and other regions in need of the cooling ice. They are distrusted by most of the sects of the Imperial Creed due to there claim that on there home is a massive iceberg that contains a shard of the diads power that can purify esoterics, and there claim that icebergs exist on other worlds they control that reflects this light.
This house is tolerated due to its nessisity, its wealth, and there skill at purging heretical cults. They are experts in rooting out cults, even the extremely insidious cults of Tzeentch and the Tyranid vanguard organisms. In combat, they employ a large number of esoterics with powers such as channeling the wind of Frijir and cyrokinesis. There standard weapon is an anti-laser variant of the lasgun, which draws mechanicus suspicion due to the anti-laser being heavily associated with the policy of Novaya Kislev and thus being heathen tech.
House Mizzen
Originating from the hive world of Chaddis in the Brine Galaxia, House Mizzen was born in the wake of a devastating civil war only stopped by an invasion by a tendril of the Tyranid Splinter Fleet Charybdis. The heavily scarred Oxygen Guild sought unity and purity above all else. House Mizzen's eugenics programs have ensured that those of House Mizzen is not only the smallest and least widespread of the Brine Houses, but also ensures that all members will need to rely upon cybernetics to function.
House Mizzen is rich due to its dealings with imperial authorities and the mechanicus, enough to keep itself a true criminal house instead of a jumped up gang. Its main interaction with the other gangs is as diplomats and tension tamers. Though they do constantly spout propaganda and kill undesirables, those under the control of House Mizzen can work and live without fear of random assault and thievery, a rarity in the Imperium.
Howling Dead Men
It is said that fascism keeps the trains running on time, and for the highly fascist Imperium of Man this is true, if only due to the Howling Dead Men.
This gang has the unique honor of originating on the planet of Brine itself, instead of just from the Brine Galaxia. The oldest of the Brine Houses, it began soon after the strange flooding of Brine and most of its worlds, they constructed a massive network of undersea and underground trains and monorails so that others could travel around Brine with ease. They would spread the farthest of all Brine Houses, and indeed of all criminal houses. There are Howling Dead Men gangs on Holy Terra to as far as Domain Novus and Imperial Worlds in the Koronus Expanse.
All train stations under the control of the Howling Dead Men are so orderly and safe that neither arbities or ministorum need interfere, and rival gangs use them as neutral areas. The trains themselves are bedecked with wards armor, and defensive weaponry to ensure the protection of the cargo, with even Orruk Speed Freakz and Eldar Harlequins having difficulty attacking the trains of the Howling Dead Men.
The forces of the Dead Men are armed with Needle Guns and Crow Bars, which often prove effective against those who manage to board the train, there faith in the Imperium enabling them to harm even insubstantial foes like Nighthaunts or Daemons. They are never without a sealed set of power armor enabling them to function in hostile territories like oceanic depths or the vacuum of space.
So dedicated to the Imperium are the Howling Dead Men that the leaders of the gang and its myriad descendants follow the same path that the first lord of the gang did. They merge with the greatest train in there territory, the best defended and best transporting, in much the same way imperial admirals will merge with there flag ship, or titan pilots with there god machines. Though resembling servitors, these lords retain there leadership positions and direct the others to defend the train network they have sacrificed there humanity to. In many ways, they are what the imperial civilians aspire to be, sacrificing everything to a corrupt system.