The Sagodjur Fjorlag is subjected to a rapid attrition rate (and almost never the result of conflicts) which, combined with an agonisingly slow recruitment process, leads to very predictable results as far as stability in the Chapter; if an Astartes makes it to veteranhood and acquits himself as something worth keeping, he's bundled into the 1st folk and kept alive by whatever ghastly means the Chapter can bring to bear.
Kleitos, for one, has been the company captain since it was a splinter of the esteemed VIIth, but despite the visitations of his terrific age clearly lain into the remains of his face and armour (into which he has been firmly stapled) he is not necessarily the oldest surviving member of the Chapter, however much he should like to say so. The 1st captain claims his longevity comes from only drinking on wednesdays and his stubborn refusal to teleport - though this latter sentiment tends to manifest itself in the form of a phobia, rather than a lifestyle choice...
No mortal man, or even post-mortal for that matter, should live so long. Strange things tend to... appear, shall we say, when reality is burdened with a mind so vast and ancient, or with a body so far decayed and reborn through sheer froce of will. Pay it no mind should you think you see anything amiss, you don't. Honest.
the gunslinger baronies
When the Nastrondites were evacuated from the dying remnants of their homeworld, they left behind domestic law and order, and in its vacuum was sucked in anarchy instead. Millions of ungovernable citizenry, crammed into the black gullet of a quasi-world they could not comprehend and expected to lie still like good children until their belated father returned home - what could a sane mind do except go mad?
The old Nastrond monarchy was gone. Nobility had been obsolete for centuries at any rate, and seeing as the only faction with any means of enforcing ANYTHING was the Sagodjur Fjorlag, the angels of death who had compelled their own people into the blasphemous ascension of an afterlife in the space hulk, it was natural that they alone should bother with making any rules at all.
The hive city Nastrond is broken into dozens of fifes, each fife ruled by a gunslinger baron under the direct jurisdiction of the Wuotan. Veterans of war both physical and legislative, these Astartes live in forts outside the Doom Chair, and have first choice over the aspirants badged and ticketed as coming from their fife.
In times - well, we cannot say of peace - in times then outside of war, the gunslingers’ responsibility is to maintain the fragile economy of their part of the city, and balance it between rival fifes to prevent an individual’s stake becoming too desirable. The gunslingers will stage bank robberies and train heists, engage in tourneys or duels and will essentially be at one another’s throats any hour in the day: when war beyond the stars call, however, those rivalries are left behind. For ultimately the gunslinger barons are brother Astartes, to whom their domestic life seems to be lived by another being.
How bizzarely do the Sago Astartes revel in the face of discomfort.
The deeper, the more decrepit the subterene haunt, the closer the press of the cloy and the stink only serves to please them the more - for in the world-tree’s boughs that stretch beyond the bounds of human misery, there will the Sago Astartes fledge their nests. Yet amongst them there are those individuals who would go to lengths to find in that depth a deeper still; be it for personal validation or even deprecation, and it is they who will wander themselves into the Sumpmarine ranks.
The uniform of the trade is plastic wrapping to save from befoulment the arms and artifice that is worth more than the Astartes who bears it: this, and whatever means of filtering out the absolute, gut-churning stink of the sewer plants complete the Sumpmarine’s gear.
Ah, you might ask that. Why would the Chapter direct a hive’s sewage into the roots of its own fortress-monastery? But would that not rot them from within? Perhaps it’s some towering contraption of a more masochistic bent? Perhaps. But there is reason besides that.
Despite beurocratic liaising and loopholes - despite any lengths to which fickle Imperial ignorance can stretch - Nastrond is, and will remain, a space hulk. And if the millennia intervening has taught one thing to the commander confronted by such an abomination, let it be that they are never untenanted. Mount Nowhere is not exempt, fortress-monastery or no, from those things that crawl noisomely from the quasi-corners and the worm-holes burrowed and melted and rotted away through the bones of the colossus; this is not even a secret the Chapter bothers to keep anymore, few though they have.
But acceptance of their existence does not make the interlopers welcome, of course not. They are funnelled down through the interlocking sewage plants, out of the reach of cities to where best the chapter can isolate and eradicate - and so, with all that hazard and unmentionable horror brought into a singular, multifarious point, the Sumpmarines are at liberty to destroy all they can find and thus preserve Nastrond’s people.