The Shadow Marches are not a recognized nation. Rather, the Marches are an independent territory whose established relationships with its neighbors and the outside world comes largely from the presence of House Tharashk. The folk of the Shadow Marches have a reputation for xenophobia, and outsiders are rarely welcomed here.

Orcs make up more than half the population of the Marches. Humans and members of other races also dwell here. The Shadow Marches feature two distinct cultures—the orc tribes that settled these lands, and the clans formed by later human migrants.

The orc tribes are an insular society, but humans are sometimes accepted into an orc tribe if they can prove themselves. The tribes are territorial, and typically attack trespassers on sight.

Closer to the outer edges of the Marches, mixed clans of humans and orcs are more accepting of the outside world, mostly through their association with House Tharashk. The clans have more in common with the folk of “civilized” Khorvaire than do their kin in the orc tribes, and they demonstrate a greater level of trust toward outsiders.

House Tharashk originated in the Shadow Marches, and the house retains strong ties to this land. Today, Tharashk makes a majority of its profits shipping Eberron shards from the Marches to the rest of Khorvaire. The swamps of the Marches contain some of the largest Eberron dragonshard fields in Khorvaire, and Tharashk operates prospecting and distribution centers throughout the region.

The Mark of Finding appeared shortly after humans came to the Shadow Marches 1,500 years ago, but the sheer isolation of this land meant that it remained unknown for centuries thereafter. Only when explorers from House Sivis came to the Marches was the Mark of Finding discovered. Subsequently, Sivis gnomes shepherded the humand and orcs of this realm through the process of creating House Tharashk. The Mark of Finding is never found on humans, but they have remained an important piece of House Tharashk's operations.

The first humans to enter the Shadow Marches were not native Khorvairians, but a second wave of refugees from Sarlona, fleeing east across the Barren Sea. Over long years, conflicts between these migrants and the native orcs slowly subsided. However, it was the war with the daelkyr that forged a lasting bond between both races.

The daelkyr incursions laid waste to these lands, and ruins from that long-ago conflict are still scattered throughout the swamps.

Politics of the Shadow Marches

The wetlands of the Shadow Marches are defined by their resiliency, withstanding the tests of time and weather. While not as primeval as the jungles of Q'barra, the marshes of the Shadow Marches have avoided the transformative works of empire, both from humans and goblinoids. The Dhakaani did take an interest in the region, but their outposts were few in number and isolated in impact.

The House Sivis map most members of the Five Nations are familiar with does a poor job of representing the nuance and details of the marshes, papering over the waterways with a simple swamp texture. The Shadow Marches are full of small lakes and rivers, with many of the latter flowing into the Glum River. House Sivis cartographers have only recently grown interested in the headwaters of the Glum, and were surprised to discover that the region's rainfall wasn't able to account for the size and volume of the river. Rather, underwater springs provide a seemingly endless supply of water, causing some to question how deep the water goes and if it might link to an aquatic realm of Khyber.

The Seasons

The Shadow Marches are defined by two dominant seasons - the wet season from Dravago (May) through Rhaan (September), and the dry season from Sypheros (October) through Eyre (April). During the wet season, warm winds come in from the Barren Sea, saturating the region. Tropical cyclones blow in from the south, with fierce winds that demolish coastal trees. While severe winds can make fishing dangerous, the water provides for bountiful agriculture.

During the dry season, cold air comes down from the Demon Wastes and blows over the Shadow Marches. Greatly diminished rains require the inhabitants of the Shadow Marches to stockpile food for the winter months for them and their livestock. Fortunately, colder and drier weather also suppresses vectors for disease like insects.

Terrain

More than any other biome, wetlands are full of treacherous terrain that can make for dynamic and risky combat. The general lack of solid ground makes boats the quickest and most reliable method of travel. Wealthy groups will hire experienced guides to paddle for them, while more novice parties may be forced to choose between keeping moving the boat and drawing their weapons.

Amphibious ambush predators, whether a mundane crocodile or a rogue black dragon, are particularly frightening when they combine standing water with poor visibility. Not only can these creatures use the environment to get the drop on potential prey, but water provides excellent protection against attacks. Melee attackers find themselves completely out of reach unless they choose to brave the murky depths, while many ranged weapons are less accurate underwater.

Even when there is ground, there's no guarantee it's an adventurer's friend. Thick mud can slow a veteran swordsman, while tall reeds and trees can provide cover and concealment against ranged attacks.

Settlements

Despite the progress by House Tharashk to unite the Shadow Marches, the region still lacks basic infrastructure like trade roads to promote interconnectivity. House Orien Trailblazers have attempted to tame the marshes but have made little progress in finding or creating durable pathways. Boats remain the most reliable method of navigating the waterlogged marshes, but House Lyrandar has been slow to invest in the region, their seafaring vessels unsuited for shallow water.

The orcs of the Shadow Marches live in elevated housing built to accomodate even the highest of tides. While the wooden construction may give the appearance of poverty or even savagery, druidic magic reinforces the stilted buildings to resist wind and water. Ceilings typically have two layers separated for ventilation, allowing an escape for high winds that blow through the structure while still providing cover from the rain. Most buildings consist of only a few large rooms, relying on posts and beams for internal support rather than load-bearing walls. Still, even with help of magic repair and reconstruction is a fact of life.

The Inland Marches

Traveling north and east, the wetlands of the Shadow Marches give way to forests and plains. The intense storms that wrack the coast dry out and dissipate. The distance from Zarash'ak means few outlanders have ever stepped foot in the Deepwood, where orcs live in sturdy wooden housing.

The southern peninsula nestled between Zarash Bay and Azure Bay holds Balinor's Sorrow, a dangerous wood filled with dire beasts. Morgrave scholars have speculated a Lammanian manifest zone here helped the Gatekeepers breed the first horrid animals, adding chitinous armor and acidic natural attacks.

Along the northern coast bordering Crescent Bay, House Tharashk has established Yrlag as a safe backup from which to launch expeditions into the Demon Wastes, resupplying the exploratory village/forward base of Blood Crescent. Some tribes maintain the traditional nomadic lifestyle, while others have taken up agriculture to trade with Tharashk.

Eberron Campaign Guide

The Shadow Marches are a vast swampland teeming with insects, reptiles, and vermin. The territory contains little dry land and no trade routes. Settlements in the Marches are typically constructed on stilts and pilings to keep them above the shifting water levels of the swamp.

Though much of the Marches’ swampland is shallow enough to wade through, virtually all travel here is by boat. Predators, disease, and other hazards make finding food and drinking water here more difficult than in other wilderness areas.

The richest dragonshard fields in the Marches can be found on the western bank of the Glum River between Glumtown and Zarash’ak, in shallow water. Such finds are marked by a pale red glow beneath the muck.

Keith

http://keith-baker.com/fens-and-marches/ 

As such, you could easily incorporate the most distinct physical feature of the Fens into the Shadow Marches. These are the Titans: trees which once grew up to a mile in height, but which were struck down in some ancient cataclysm. Their wood is infused with magic that prevents decay. So although the trees are long dead, but they form the physical foundation of the swamps. If you embrace this idea, the clans and House Tharashk carve their cities into the stumps and trunks of the Titans, while the tribes generally live atop them or make use of natural cracks and crevasses in the surface of a Titan. Both groups harvest lumber from the Titans, though the tribes approach this in a more industrial manner; this process is more akin to quarrying stone than the work of the traditional lumberjack. In d20 terms, the wood of a Titan would generally be considered to be Densewood, with veins which if harvested and treated properly can yield Bronzewood (both materials described on page 120 of the 3.5 Eberron Campaign Setting). In canon Eberron these rare woods come from the forests of Aerenal, but it’s not particularly unbalancing to give these resources to the Shadow Marches… and it justifies Gatekeepers having ancient bronzewood weapons and armor dating back to the Xoriat incursion. While you could make this one of House Tharashk’s industries, I’d be inclined to have Tharashk keep its focus on finding rarer things. Densewood-grade lumber could be an industry that the clans focused on before the rise of Tharashk, while Tharashk uses the Mark of Finding to locate the rarer veins of Bronzewood.

Aside from creating an additional industry for the Marches, this has a few effects.

• The clans live in fortified communities, carved into the natural shelter of the Titans. Tribes or more isolated families will live atop Titan trunks or in natural “caves.”
• The people of the region use wood for things that would be made from stone or steel in other places. If a building isn’t carved into a trunk or stump, it will be made from wooden blocks. Wooden spears are very common — used both for defense and as walking staffs — and knives and swords are typically made of Bronzewood.
• The fallen Titans create a network of islands in the swampy morass. In heavily trafficked areas, bridges connect these islands; beyond this people generally use small boats to get from place to place.
• The Titans add a vertical aspect to the landscape, especially as people generally live atop them or in their trunks. Bear in mind that the Titans fell thousands of years ago, and many have layers of soil and vegetation that have built up on their trunks.
• In the Marches/Fens, the Titans have all fallen. However, in Eberron it is possible that living Titans can still be found. The most logical location for this would be the so-called Towering Wood in the Eldeen Reaches. You’d have to decide if the trees of the Towering Wood are full-sized Titans, or perhaps a similar but smaller variant. If you do have Titans, the next question is if one could be awakened. A human is essentially an ant to a Titan, which would make interaction with a Titan difficult. Even speak with plants might not bridge that vast difference of scale; if the Titan noticed the druid they could understand them, but they are still a tiny speck with a tiny voice. Given this, it could be interesting to have a single awakened Titan that’s wandering around the Reaches. Humans have no way to speak with it, but if necessarily Oalian himself might be able to communicate with it.

Eberron Campaign Guide

The Shadow Marches are the wildest corner of Khorvaire, and laws are an unknown concept here. Zarash’ak is the only exception, largely as the result of House Tharashk needing its business to run in a smooth and orderly fashion.

The Shadow Marches are not a nation, and this territory has no central government. Other nations or business concerns looking to deal with the Marches inevitably deal with House Tharashk. In the same way, nothing prevents Tharashk from exerting whatever influence it sees fit over the local tribes and clans.

Anyone who hopes to do business in the Marches— or to stay alive while traveling here—knows to keep on the house’s good side.

Politics of the Shadow Marches

The orcs of the Shadow Marches do not have a central government. Unlike the halflings of the Talenta Plains, the orcs were not forced to confront armies marching across their lands. Without an external pressure, the orcs never united to demand representation at Thronehold. While House Tharashk has developed and expanded the dragonshard trade over the past century, their influence is currently limited to the immediate area surrounding Zarash'ak and the dragonshard fields extending west of the stilted city. Outside that zone of influence, the multiracial clans and the traditional orc tribes hold sway.

Most orcs in the Shadow Marches live in small villages. Each family in the village has a parcel of land they own and tend to, growing rice and tending to a small herd of water tribex and other large animals. House Vadalis has begun to send heirs into the Shadow Marches to explore business opportunities and build relationships, however their success has been limited to clan villages as opposed to tribe villages.

Clans

Long ago, after Lhazaar's famous expedition across the sea that now bears her name, a second set of humans left Sarlona. Fleeing the expanding empire of Riedra, they crossed the Barren Sea and landed on Khorvaire's west coast. While many unfortunate souls landed in the Demon Wastes and became part of the Carrion Tribes, others landed in the Shadow Marches. Many orc tribes welcomed the refugees with open arms, integrating their tribes to become the clans. From these clans came the Jhorgun'taal, the children of two bloods.

The clans celebrate their half-orc as proof that humanity and orc kind are in fact one. This vaunted status means Jhorgun'taal hold a disproportionate number of diplomatic and advisory positions within their clans, bridging not only internal divides but building relationships with the outside world. While half-orcs, like half-elves, are a true-breeding race, unlike half-elves they prefer to remain integrated with their parent communities to forming their own.

The clans of the Shadow Marches primarily live on the coast. Having accepted human immigrants millenia ago, the clans freely incorporate and adapt tools and teachings from the outside world. However, not every technology developed for the heartland of the Five Nations works well in the wetlands of the Shadow Marches, where storms can damage or destroy imported goods. Still, the clans tend to be welcoming and inclusive of outsiders.

Few towns exist in the Shadow Marches, but those that do are almost always populated by multiple orcish clans. These towns function as regional hubs, trading supplies with the local farmers and prospectors. While most have some form of inn for travelers they're rarely Ghallanda-licensed. Despite the growing dragonshard trade few people from the Five Nations venture into the Shadow Marches, leaving little demand for hospitality services.

Clans are governed by elected councils, with the exact rules and traditions varying from clan to clan. Councils appoint reeves to carry out the law on their behalf. A reeve's executive authority is extensive, although their power is limited. However, aggressive actions taken against a reeve are liable to draw the wrath of the whole clan.

Tribes

Insular at the best of times and xenophobic at the worst, the orc tribes are predominantly found inland in the Shadow Marches. All prefer to follow older traditions, rejecting "advances" from the eastern nations in favor of known practices. Tribal traditions are untainted by Sarlonan human beliefs and rituals, meaning they have the most accurately preserved druidic and cult practices.

Tribal councils consist solely of the eldest orcs of the tribe, their wisdom and age presumed to provide the best governance. Orcs who contest the council are forced into exile, typically joining one of the clans in the region.

When faced with external conflict a tribe has a number of options. When dealing with a known tribe or clan the council of elders may directly meet the opposing representatives to work out a solution. Unknown threats are confronted by an appointed reeve and a select group of warriors. While the tribes do not usually intend to escalate conflicts, the reality of the marches lack of central governance means that tribes are ready to defend themselves.

With no history of conquests or nation-building, the Shadow Marches contain little in the way of treasure for adventurers to plunder. However, the Marches are home to some of Khorvaire’s richest Eberron dragonshard fields. Though the Marches’ larger shard finds have been marked and cataloged by House Tharashk or the local tribes, undiscovered fields still lurk in the deeper swamps for those explorers brave enough to seek them.

As new shard fields are discovered in the Marches, House Tharashk builds permanent outposts from which it can stringently oversee and control its prospecting operations. Dozens of small working camps are spread throughout the swamps, each consisting of a handful of communal huts. Larger numbers of former outposts now stand abandoned and rotting where the local shard fields were cleaned out. From these scattered sites, dragonshards are gathered and shipped back to Zarash’ak on well-guarded boats.

House Tharashk does not take kindly to outsiders tripping through the middle of its shard prospecting operations. Travelers can seek shelter in a mining camp, though they should be prepared to explain their business in the Marches. An abandoned Tharashk outpost might serve as an excellent temporary base for exploration, but a good number of these former mining camps have already been taken over by the dangerous denizens of the swamps.

When the daelkyr came to Eberron at the end of the Age of Monsters, the area that would become the Shadow Marches was overrun. The rune-scribed ruins of the unearthly structures that the daelkyr and their servants constructed here can still be found within the swamps—sometimes used as foundations for the huts of unsuspecting villagers. The Unknown shun these sites, but they are revered by the Cults of the Dragon Below. They might hold information important to historians, or could reveal treasures lost for millennia. Elsewhere, the foul creations of the daelkyr might still lurk beneath the mire.

Eberron Campaign Setting

The Eberron dragonshard fields of the Shadow Marches are among the richest in Khorvaire, and House Tharashk makes most of its profit shipping shards to the interior of Khorvaire. This commerce has resulted in an alliance with House Lyrandar, since that house maintains the shipping lanes between Zarash’ak and other cities, much to House Orien’s disappointment.

In addition to Eberron dragonshards, a variety of rare herbs and plants grow in the depths of the marshes. Of interest to alchemists and wizards in Aundair and Zilargo, these resources have quickly become valuable as secondary exports.

Politics of the Shadow Marches

The Mark of Finding first appeared on half-orcs while Galifar was uniting the Five Nations. The final dragonmark to manifest, the Mark of Finding wasn't known to the wider world until a House Sivis expedition ventured to the westward edge of the continent 500 years later. The Sivis gnomes quickly helped the dragonmarked clans come together to form House Tharashk and its ruiling Triumvirate.

The Triumvirate

While House Tharashk is made up of many clans, it is ruled only by three - the Aashta, Torrn, and Velderan clans, who have ruled the house jointly since its inception. The current triumvirs are Daric d'Velderan, Khundar'aashta, and Maagrim Torrn. The Mark of Finding is not a requirement to serve even in the highest leadership positions, only competence and business sense. Below the triumvirs the house is structured traditionally with viceroys, ministers, and masters ordering the house's business within regions.

The Aashta Clan

The most sinister of the three major clans, the Aashta have numerous ties to aberrant cults of the dragon below. Many of these old traditions are fundamentally harmless, even if disturbing to outsiders. Profane prayers and symbology litter Aashta clan traditions, but finding ones that actually manifest dark influence is akin to finding a needle in a haystack. Those families within the clan that have true connections to servants of the Daelkyr use their position to acquire dragonshards for eldritch rituals.

The Torrn Clan

Typically quiet and reserved, the Torrn clan has deep alliances with the Gatekeeper druids. The clan's elderly matriarch Maagrim Torrn is herself a practicioner of the druidic ways and makes efforts to support the maintenance of the seals throughout the Shadow Marches. On more than one occassion she has bought out a dragonshard field from another clan to block its extraction so as to avoid disrupting one of the seals.

The Velderan Clan

Good-hearted and innovative, the Velderan clan has been the most forward in adopting the styles and traditions of the Five Nations. The clan bears no particular ties to the Gatekeeper druids or Khyberian cultists, although some families within the clan do follow one or the other (just as not every member of the Aashta and Torrn clans follows the predominant faith of their clan). The discovery of massive dragonshard reservoirs in Q'barra a decade ago was made by a Velderan enterprener. The Velderans are also the most diplomatic of the major clans, bridging arguments not only between the Aashta and Torrn clans but between the House and the inland tribes when the House expands its operations.

Business

House Tharashk began with bounty hunting but quickly expanded into inquisitive work, the mark of finding placing them into direct competition with House Medani. Persons hiring a Finder's Guild inquisitive do so with a specific purpose, as opposed to the higher-cost general threat assessments provided by a Warning Guild inquisitive. This is less useful to the wealthy, who tend to have many needs and are particularly concerned with uncovering unknown unknowns, but House Tharashk has made up for this by opening their doors to the common man who needs a professional for a specific purpose.

A particularly enterprising gnome in the seventh century brought Tharashk operatives to Zilargo to survey for minerals. To her delight, the Mark of Finding proved fruitful, establishing a link to the wealthy of Khorvaire through their prospecting business. Despite this, House Tharashk has been slow to leverage these opportunities.

Over the past two centuries the prospectors of the Finder's Guild have grown from a fledgling industry to a majority of the profits. Advances in magical technique have transformed dragonshards into a general commodity that can substitute for numerous other rare and costly magical components. With the fracturing of Galifar this makes dragonshards a uniquely universal currency. While the house sends plenty of expeditions into Xen'drik for Siberys shards or into the numerous mountain ranges for Khyber shards, the Eberron shards in the Shadow Marches have remained the most consistent and profitable source. Even the past decade in Q'barra has been exciting for its potential, not its current volume of production.

One of House Tharashk's oddest additions to its business has been the Dragonne's Roar. While many nations sought monstrous mercenaries throughout the last war, even before the Daughters of Sora Kell united the fracturous warlords of Droaam, House Tharashk found success where human envoys could not. Many agreements are brokered in Zarash'ak through the permanently-stationed emissaries of Droaam.

The Shadow Marches aren’t a nation, and the region wasn’t recognized by the Treaty of Thronehold. Most Marchers have little interest in the outside world. Marcher tribes don’t know the names or number of the nations beyond the swamps, let alone that they were at war. However, as House Tharashk gains power and influence in the world beyond the swamps, it has increased its presence in its homeland. The house has expanded Zarash’ak dramatically over the last decade and is working on transforming other towns into cities. This has caused tension with a number of tribes. And House Tharashk’s mining operations and its urban expansion always run the risk of disrupting Gatekeeper seals and unleashing ancient evils bound beneath the Marches.

Hidden behind Droaam, the region known as the Shadow Marches has been isolated from the events that shaped Khorvaire. The War of the Mark and the rise of the dragonmarked houses, the formation of Galifar—even the Last War itself—had little direct impact on the distant Marches. Today, the Shadow Marches are not recognized as a nation by the Treaty of Thronehold, and no central government exists there. The denizens of the Shadow Marches are split into two primary groups: those who are members of tribes, and those who are members of clans.

The tribes are primitive, and largely composed of orcs. Their traditions date back thousands of years before the arrival of humanity in Khorvaire, and many of the tribesfolk believe that humans and members of the other races can never understand their ways. As a result, they prefer to be left in solitude—and those who follow the Dragon Below can be quite dangerous if their privacy is violated.

The clans are the more civilized inhabitants of the region. When human refugees arrived from Sarlona fifteen hundred years ago, they brought Riedran customs with them. While a number of the orc tribes fought against these strangers (as some still do), many embraced the newcomers, forming an intriguing blend of human and orc cultures. While a few clans remain entirely human, the majority include humans, orcs, and their half-orc offspring. The Marchers are a proud folk, and they take bonds of blood seriously. Foreigners must be careful in dealing with the clans, for any insult to a family member will likely be taken as an affront against the entire clan.

The Orcs of the Marches

The orcs are one of the oldest races in Khorvaire, with a history stretching back some thirty thousand years. While they are not as cunning or as wise as humans, the orcs of the western coast have always been a deeply spiritual people. Even among the clans, where the traditions of humanity have had a strong infl uence on their culture, the orcs remain strong and unswerving in their faith. The majority of the clerics, adepts, and druids found in the Shadow Marches are orcs or half-orcs.

Today, the faith of the orcs is split between the path of the Gatekeepers and that of the Dragon Below. Those who follow the Gatekeepers do not necessarily know of the daelkyr or the ancient wars, but they believe in the power of nature and the duty of the orcs to remain close to their wild roots. The Gatekeeper barbarians celebrate this bond, and their rage is a manifestation of this primal connection. The Dragon Below cultists are all too often touched by madness, or believe that all power and glory fl ows from Khyber and Xoriat. The rage of a cultist barbarian is a terrifying thing—pure madness concentrated into martial fury.

Orcs of the isolationist tribes believe that humans and orcs have no common ground because orcs were the first inhabitants of the land, and only an orc can truly bond to Khorvaire. However, the orcs of the clans see humans as their brothers and sisters—smaller and weaker, perhaps, but valued for their cunning and quick wits.

Life on the Frontier

The Shadow Marches have a larger human population than Droaam, and considerable wealth can be found in this land—from vast fields of Eberron dragonshards to ancient treasures hidden in the ruins of the daelkyr. Furthermore, it is far from the reach of the laws of other nations. A fugitive on the run from the King’s Citadel can find security in the shadows of Zarasha’ak without giving up human company. The Shadow Marches have never been bound by the Code of Galifar, and even the sentinel marshals have no authority in this region.

Even as this isolation provides security for the fugitive, however, it can be dangerous for the adventurer. There is no unified law in the land, and each clan or tribe rules its territory as it chooses. Most Marcher villages have a sheriff who holds the full authority of law. If adventurers challenge or kill a sheriff, they have committed a mortal crime against his people—a crime that calls for blood vengeance from them and any of their allies.