Medieval doctors had limited empirical knowledge and scant instruments, but they made up for their shortcomings with a brazen willingness to expound elaborate theories. Among the biggest medical questions of the Middle Ages was the cause and nature of the bubonic plague. The explanation varied tremendously in particulars, but the broad consensus was that the plague was caused by foul air and water, often corrupted by "earthy bodies."
Under certain alignments of the stars, the earth would release putrid gases and liquids that would be distributed by wind and cause the plague wherever it arrived. This plague gas could also emerge from more localized phenomena--rotting food, bodies, dank caves, manure, industrial waste, and tanneries--but plague doctors returned again and again to the idea that the ultimate source of the plague was the depths of the earth.
Earthquakes of course emitted noxious exhalations from deep within the bowels of the earth up into the air, producing an "eternal night" and generating "new kinds of diseases" that come about from the "many poisons that arise therein, spread not by [human] hand but spontaneously."
Earthquakes can and do release noxious and odorless vapors that can suffocate and poison humans and animals. And rotting corpses and other things that smell bad are likely to contain pathogenic microorganisms. The plague doctors err mainly in that they ascribe all these phenomena to a single category and a single source. In doing so, they create an abstraction, interchangeably known as miasma, putridity, corruption, poison, foulness, badness, rot, or stench, which mirrors the way that games generally handle such concepts.
Avicenna connects all three elements-air, water, and earth-to disease by explaining that sometimes pestilences arise from "putridities" that occur "in the bowels of the earth" that then "cause harm to water and air" but that the reason for why this happens is unknown."
Medieval plague doctors advanced all sorts of arcane theories on minimal evidence, but apparently the source of subterranean corruption was the one topic they were not prepared to speculate on. My source on the topic, John Aberth's Environmental History of the Middle Ages, quotes plague doctors extensively but makes no reference to this seemingly obvious question. The fact that the plane they inhabited, designed for them by God, yielded poison gases whenever pierced seems like a noteworthy fact of theology and mythology. It is an odd fact to take for granted.
Konrad of Megenberg is the scholar most dedicated to the earthly explanation. He describes the plague as "a corrupt and poisonous exhalation from the earth, which infected the air in various parts of the world and when breathed in by men suffocated them to the point of sudden extinction." The terminology evokes ecology--the earth is alive; it exhales poison like trees exhale oxygen. What is this being, lurking in the bedrock, spewing bubonic halitosis into the medieval landscape? Did this not spur the medieval imagination in the way it does mine?
In a post-Lovecraft world, medieval plague theory is a tight-lipped invocation of an evocative premise. Our world is full of vast and ancient poisons, and our health is at the mercy of unknowable machinations of star patterns and the motions of chthonic monstrosities. Reza Negarestani's Cyclonopedia is a geopolitical version of the concept, in which oil is a sentient entity and its extraction and distribution initiates some vaguely apocalyptic circumstances.
From Software's Dark Souls 2 provides a near-perfect cosmic horror elaboration of medieval plague theory. The first Dark Souls is rich in delving, covering some classic chthonic tropes like the demon volcano city, the skeleton catacomb, and the dark abyss. But Dark Souls 2 ties a list of plague tropes and motifs together to create a thematically linked environmental arc, an arc that plumbs even deeper chthonic depths than its predecessor.
Harvest Valley
In Harvest Valley, a vast tower studded with giant windmills pumps poisonous liquid from the earth. Trenches and pits line the outskirts of the tower, flooded with green miasma. It's not clear what exactly is being harvested. Nothing grows here; the earth itself is fouled. A merchant sells minerals, and the shallow trenches are called The Mines. Aberth tells us that "Windmills, especially toward the end of the Middle Ages, were to develop other applications besides grain and industrial milling, such as pumping out water in order to drain the . . . ever deeper mines of Germany." Elsewhere he advises us "[not to] forget mining, since this was mentioned by Konrad of Megenberg as giving access to deadly vapors or fumes trapped within the earth." Perhaps the windmills pumped the poisonous fluid from the trenches to facilitate mining titanite and other stones?
But no. Laddersmith Gilligan tells us that "there's this monster lady, right . . . and the poison, well, does wonders for her body . . . you know, health and beauty, and that sort of stuff." So it seems that the poison is the product. And this is borne out by the evidence; the tower is filled with enormous jars of the stuff. Mining for poison is not a fantasy in the Middle Ages: mines produced both lead and arsenic as marketable products.
Down the Pit
The poison at Harvest Valley is not a byproduct of mining or some curse that befell the land. It is "found deep within the earth." There is no proper mine at Harvest Valley. The entrance to the depths of the earth is instead found in Majula. The town is built around a structure known as the Pit, which appears to be an enormous well, though this is likely not its intended purpose. Regardless, the well is another classic motif of the plague theorist:
"One [of Megenberg's proofs of an earthly origin for plague miasma] is that air, when it is 'shut up and imprisoned for a long time in the earth becomes so corrupted in some corner of the earth that it may be turned into a potent poison to the human constitution.' This is demonstrated . . . by the fact that wells, when they are first opened after being sealed and unused 'for several years,' typically suffocate with their fumes the first person to go into them."
Within the Pit there is a rat-infested crypt. Rats are the most prominent symbol of the plague to modern audiences, but medieval doctors would have been more concerned with the corpses than the rodents. In fact, some doctors were more interested in whether rats were catching the plague than if they were spreading it. Avicenna indicated that mice and rats fled the earth in advance of a spreading wave of underground putridity. Whether other creatures were susceptible to the same diseases as humans was a topic of much debate, but since all poison and disease were of a single essence, the general intuition was that they were, except in the case of inherently venomous organisms.
The King of the Rats begins to elaborate some of the chthonic mythology of the Pit.
Long ago, I struck an accord with a human chieftain. 'Twas agreed, Humans would rule the lands lapped by the sun's rays, and rats, all that was below. But humans are liars all, schemers and cheats. Humans are creatures of greed. The time came they broke their word, and hunted us down.
He goes on to refer to humans as "sun-poisoned," implying that indeed there is in fact a dichotomy of poison natures. The chthonic putridities are their natural home, and our home is as foul to them as theirs is to us.
The Gutter
Further down the Pit, far below the Rat King's crypt, lies the Gutter. Laddersmith Gilligan tells us the Pit is "where the people round here toss all their rubbish. Corpses or you know what… Whatever they'd rather be rid of." In medieval cities, trash, including all the muck and nightsoil people washed into the streets, was a major source of stench, and cleaning it up was one of the few suggestions plague doctors made that was both tangible and productive. The Gutter manifests this public health nightmare as a vertiginous cesspool, a teetering frame of timber rising upward in a cavern oozing slime. Pillars of trash, corpses, sewage, and cabinetry descend from the upper reaches.
If the link between this sewage and the broader theme of the Pit seems tenuous, note that poison in DS2 can be inflicted as well by a lump of shit or spoiled pine sap as by a poisoned blade or cloud of gas.
The Gutter is dotted with poison-spitting statues, a chorus of pulsing vessels that fills the cavern with an eerie disembodied heartbeat.
A tenuous network of rickety ladders drops to the bottom of the Gutter and into Black Gulch.
The Rotten
Black Gulch is spattered in a bright green lichen that evokes the miasma of Harvest Valley. The poison spitting statues of the Gutter are at their most dense here, and they too are coated in the green lichen. Tar pits and holes in the wall are inhabited by creatures that are, once again, unperturbed by poison.
At the bottom of the gulch lies a monstrous agglomeration of past-its-prime flesh called The Rotten. It is a golem of corpses, grotesquely bloated and moist compared to the game's typical denizens, who appear mummified (that's my character, Pubic Shout, on the right, for comparison). Medieval doctors were concerned that plague would spread from "rotting cadavers in cemeteries or left on the battlefield during wars"--The Rotten is an incarnation of their worst nightmares, if they had them. And again, if the environmental links between these corpses and plague/poison in the game are too circumstantial, note that poison can also be inflicted by "germs on [a bat] cadaver."
Is The Rotten the source of the plague vapors boiling up from the earth? It makes a strong candidate. It has the charisma and the potency to putrefy the world. It even has the soul of an Old One.
But delving is never done. Beyond the Rotten a fountain of coiled, headless serpents offers access to a hidden city that lies even deeper inside the earth.
The Poisoned Dragon
Shulva, the Sanctum City, is a massive Mayan temple within the biggest cave of the Pit. Devotees of the earthly hypothesis of plague formation generally considered caves sufficient sources of miasma on their own, waiting only to be released by earthquake or mining. But the source of Shulva's poison is more concrete.
In true Lovecraftian fashion, the denizens of the Sanctum City worship and protect a sleeping dragon, Sinh, with poison in its heart. The priestesses of Shulva sing the dragon to sleep, ostensibly in its service but implicitly to protect the city from its ravages. Graverobbers coveting the dragon's blood, the Drakeblood knights led by Sir Yorgh, invaded the city from above and fought their way to the center of the Sanctum, Dragon's Rest. Sir Yorgh managed to thrust his giant spear into the slumbering dragon's heart.
When Sir Yorgh faced Sinh, the slumbering dragon, he drew blood with a flash of his steel, but Sinh responded by spewing forth the poison that had long brewed within him, blanketing the city in a miasmic cloud.
The only men who remain in Shulva are suffused with poison, their bodies oozing foul vapors. It is plausible to imagine that the miasma released by Sir Yorgh drove the inhabitants of Shulva up into Black Gulch--supplying the corpses of the Rotten, perhaps? Some may have even made it to the Gutter, where their belongings were piled up and scaffolding constructed in a desperate effort to reach the surface and escape the miasma (this hypothesis comes from this Reddit post). The game encourages the conclusion that that Sinh was the source of all the corruption in the Pit. He sleeps in "the lowermost depths of this land," preempting the idea that there is some further source to be found still lower in the earth. Yet there remains a tantalizing clue:
The King erected a magnificent city, and the dragon slept soundly. Until Sir Yorgh disturbed it with a single great strike, and the dragon could bear its store of poison no longer.The rain of death toppled the city, but restored the dragon's purity.
Conclusions
If lancing the dragon and releasing its festering poison restored its original purity, then the corruption must not be endogenous to the creature itself. Sinh is not a being of poison; he was poisoned himself by something else. On the ultimate source of that corruption, the game gives no further hints, remaining as mum as the plague theorists themselves.
This line of reasoning is sort of just an intellectual curiosity. The conflation of poison and disease into a single status effect category, essentially a new element, is common to a lot of games; what makes Dark Souls 2 unique is its willingness to dive into the chthonic mythology underpinning that element. It is a difference of degree more than kind in its affiliation with the medieval plague theorists.
It seems fairly certain that the parallels were not conscious on the part of the developers. The alternative explanations, I think, are either that these plague tropes were codified and collected by the plague theorists and advanced together in the public imagination ever since, or that these disparate forms of badness and their chthonic roots are intuitively linked in the Western imagination.
I think that some of each is true. There seems to be a general human tendency to dichotomize dirt/cleanliness, where dirt is a catchall for anything corrupt, dangerous, or unpleasant. But on the other hand, chthonic mythologies in other cultures--see the Greek or Egyptian underworlds, for instance--never mention poison at all, so this idea may be unique to medieval Europe.