Black Marsh, also known as Argonia, is one of the nine Provinces of Tamriel. It is the home of the Argonians, the reptilian natives of this swampland rainforest. The province is filled with "great inland waterways and impenetrable swamps" . The Dunmer of neighboring Morrowind used to enter the region for slaves, particularly House Telvanni. Some time after the Nerevarine left Morrowind, slavery was outlawed.
There are several possibilities that could be the ruling body on Black Marsh. One of possibilities is the An-Xileel who seemed to be in charge of Black Marsh after the Oblivion Crisis. Possibly a political faction in Black Marsh that was part of the invasion of Morrowind. 
Since there are references to the King of Argonia and an Argonian Royal Court, this might be the central governing body.
Shadowscales are Argonians, born under the sign of The Shadow in Black Marsh. They are trained from childhood in the art of stealth and combat. Shadowscales serve their country as assassins. They have historically been used by the King of Argonia as a means to end conflicts and wars without major bloodshed. 
While it was under Imperial rule, Black Marsh was under the protection of the Imperial Legion. The Legion works for the Imperial government, with support from the Emperor. In peacetime, the legionnaires serve as guards; in war, they have been used as an invasion force. In battle, the Imperial Legion is a feared and unstoppable force. Their huge numbers and strict discipline cause even the most battle-hardened army to question their tactics. With wide ranks of hundreds of disciplined soldiers, the Imperial Legion is a force not to be trifled with. However, the thick forests and dangerous swamps of the province typically stave off any invasion attempt. In addition, the Argonians natural adaptiveness to their homeland has caused many invasion attempts to fail.
Little is known about what occurred in Argonia or Argonians in general during much of the First Era, as the few Argonians that left the province were very rare and they themselves didn't talk about their history and preferred to integrate themselves into the larger Tamrielic cultures. The region and, by extension Argonians themselves, stayed independent of the Tamrielic Empire until the twenty-ninth century of the First Era.
It is known that there was a very successful enterprise of bandits and thieves that had long been exploiting the swampland of southeastern Tamriel, which was lead by most infamous of the brigadiers, "Red" Bramman. The coastline along the east of Topal Bay had become notorious for acts of piracy because of these Bandits and thieves, and in 1E 1033, the Empress Hestra demanded the head of Bramman.
After many an unsuccessful battle in the Bay, the Imperial Navy discovered the pirate-king's means of escaping capture: a narrow, winding river which emptied into the Bay near Soulrest, its mouth was screened by dense thickets of mangroves. The Imperial Fleet followed the course deeper into the heart of Black Marsh than any non-Argonian had ever been before. They eventually caught Bramman in his bandit kingdom not far from what is now called Blackrose, and took his head for the Empress.
The Argonians were defeated by the Reman Empire in 2811 at the Battle of Argonia. Following this defeat, the Argonians retreated to Helstrom, where the Imperials would not follow them. Slave traders exploited the Argonians, and "entire tribes of Argonians were dragged in chains" to the Dunmer land. Warlord dynasties were formed by former Imperial officers, who "earned a reputation for tyranny even in those dark times".
The land that had once been a haven for Tamriel's criminals became its greatest prison state. Anyone considered too dangerous to hold in "civilized" dungeons in other Provinces was sent to Black Marsh.
Even less is know about Argonian history in the Second Era. The only know recorded event occurred in 2E 560, when the Argonians responded against their tyrannical oppressors with the Knahaten Flu. Whether the terrible Khahaten Flu arose from natural causes, or was created by an Argonian shaman in retaliation for his people's oppression is still a matter of debate, but its result is clear. The plague began in Stormhold in 2E 560, and quickly spread to every corner of Black Marsh, killing all those not of reptilian stock. For over forty years, it held the Province in its grip, destroying entire cultures, (notably, the Kothringi), and driving outsiders from the land.
Even when the land became habitable again, fear of the disease kept most outsiders away. House Dres of Morrowind continued to send slavers into the north, but few others saw any reason to trouble themselves with the land. Even Tiber Septim, it was said, thought twice before conquering Black Marsh for his new Empire. The borders of the province fell easily to his forces, but he wisely decided to avoid strategically unimportant inner swamps, and thus met with little resistance.
By the Third Era most of Argonia had been assimilated into the empire with the exception of Helstrom. Helstrom is in the center of "Murkwood", an area of marsh in the center of Argonia where it is almost impossible for non-Argonians to travel, so is the center of non-imperial Blackmarsh. This is also where the Arnesian War uprising, (which took place during the Imperial Simulacrum), saw the Argonians extract revenge for the slavery that they suffered from House Dres.
In approximately 3E 200th year, during the reign of Katariah, there was a massacre at the village of Armanias, due to a lack of sufficient armor.
After the Oblivion Crisis, Argonia seceded from the Mede Empire, making the Marsh into an independent province. In the aftermath of the Red Year (4E 6) the Argonians invaded Morrowind to exact revenge on the Dunmer and put Morrowind in An-Xileel control. They appear to be neutral to the ongoing hostilities between the Empire and the Thalmor.
Black Marsh is one of the larger countries in Tamriel, it has an area of roughly 100'100 sq. miles. The name "Black Marsh" was given to the portion of Argonia that was conquered by the Empire in the First Era , even though Argonia had included the southern lands of Morrowind. Like Morrowind, it was "never successfully invaded, but instead, peacefully incorporated."  The majority of this province is a swampy marshland, with the southern half resembling a network of small islands, broken up by intersecting rivers. Little is known about the actual size of Argonia, except that Lilmolith is located fifteen miles from the Topal Bay.  The province of Black Marsh encompases the south-easternmost lands of the continent.
Major Cities of Black Marsh
Is a city located in the south east of Black Marsh. In 4E 187, the Listener Alisanne Dupre mentioned to Rasha, a Speaker, about reinstating training for Shadowscales in Archon. The proposal was dismissed due to a lack of resources.
A major Argonian city located in the southern reaches of Black Marsh. Notably Nienolas Ulwarth, known as the mighty, hailed from this city. Murkwood is near this city, which is a forest the Eternal Champion visits to get a part of the Staff of Chaos.
A major city in Argonia, this city was first founded by the Ayleids, and is in the westernmost section of Argonia, close to the border of Cyrodiil.
This city is in the center of Argonia. The city has never been reached by the Cyrodilic Empire and therefore is the capital of non-Imperial Black Marsh. 
Soulrest is a large city located in the south of Black Marsh where many go for their burial.
Black Marsh continues to be a "backward" land economically by Imperial standards. Most of the agriculture is grown by subsistence farmers, though recently more has been shipped abroad. The trade however is not very successful as the produce normally arrives to the Cyrodiil markets rotten, for it takes a tremendously long time for the goods to get to the market. Increased investment into improving the trade routes of Black Marsh have yielded very little success. Lord Xellicles Pinos-Revina, a wealthy trader, was the first to invest. At that time, it took three weeks, for the rice and root he was importing to arrive, half-rotten, in the Imperial Province. Now, decades later and despite millions of gold invested, the time to get to market has increased. It now takes two and a half months for goods, now thoroughly rotten, to arrive. 
The Argonians are a race of reptilian people and the most famous of Black Marshes natives. They can be found in smaller numbers throughout the continent, and have been featured in every one of the main games so far. Argonians are one of the few races completely unrelated to men and mer, being descended directly from the Hist. Enigmatic and intelligent, the Argonians are experts of guerrilla tactics, and their natural abilities suit their swampy homeland. They have developed immunities to the diseases that have doomed many would-be explorers in the region, and they are capable of easily exploring underwater locations due to their ability to breathe water.
Argonians have gills on their necks and are covered in scales. This gives Argonians them their ability to breathe underwater, which has proven to be an advantage in combat with other races, especially in the swamps of Black Marsh. Argonians are trained in guerilla warfare and drown their enemies by dragging them to the water and holding them under. Soldiers walking alongside rivers, travelling on ships in the ocean or barges along the rivers have been attacked by Argonians that ambushed them from the water where they lay in wait. They then either kill them or they are forced to flee back into the water and vanish into the depths. Either way it is an effective strategy as they cannot be followed into the deeper oceans and rivers.
The Hist are thought to share a mind or have a hive mind ability. Though they share a mind, it is possible for one to "go rogue" and "escape itself." In such instances, the Hist purge the rogue tree. This last happened to a tree in Lilmoth over 300 years ago. The current Lilmoth Hist grows from part of its root. 
The Hist possesses hallucinogenic properties when used by non-Argonian species, causing them to perceive civilians and actual enemies alike as hostile to them. Only Argonians may ingest the Hist sap to feel its full effects without negative reaction.
The Hist also seem to have the ability to communicate telepathically with people who ingest their tree sap. 
Not much specific information is known about the Argonian religion. However some knowledge about these unique beliefs are known to non-Argonians. The most important thing to understand about their religion is the close bond between the odd Hist tree and the Argonians. The Hist is a unique type of tree that grows all around Black Marsh and is looked highly upon by the inhabitants. This tree, according to the Argonians, is intelligent and fully conscious. It produces a special sap that gives Argonians visions and enlightenment to their souls. In fact, Argonian hatchlings drink the sap of the Hist at birth and infancy. According to them, the sap of the hist tree, when drank, gives a hatchling its soul. When that Argonian dies, his soul travels back to its Hist tree and is stored until another hatchling drinks that hist. Thus, the Hist is giving its a soul, and so the cycle repeats. The Hist tree can telepathically talk to the Argonian that has drank the sap.
It also should be noted that the Hist and Argonians acknowledge Sithis as their master. Along with worship of the Hist the Argonians, believe that the Sithis is the original, the creator, and the father. The Hist, in their eyes, is much like the Night Mother is to the Dark Brotherhood. To show respect and full loyalty to Sithis, since they worship the Hist as their god, every Argonian born under the sign of The Shadow is given to the Dark Brotherhood at birth and trained under the art of stealth and assassination. Therefore, one day, that Argonian can earn the rank of Shadowscale who serve Argonia as spies and assassins. 
Very little is known about the Argonian culture as of the Fourth Era. The difficult terrain within the Black Marsh, coupled with the apparent isolationist policies of the Argonians, renders this largely a mystery. It is known that great emphasis is placed upon the Hist, the strange trees that are abundant in the Marsh. Even what little is known about the Hist is the subject of much rumor and speculation. It is believed that the Argonians are somehow co-dependent upon the trees, and that the Hist forms some sort of hive-mind.  
Marital customs among the Argonians is one of the few widely-known aspects of Argonian life. Unlike the citizens of Skyrim who prefer to marry using an Amulet of Mara, some Argonians adhere to their native marriage traditions. In Black Marsh, it is customary to present a lover with an Argonian Wedding Band, which is crafted by the courting member. Each ring is unique and has personal meaning to the creator and the recipient. The actual ceremony of marriage is very long and complex, and involves unique rituals, spoken in the native language of the Argonians. 
Argonian culture in Black Marsh is not fully understood. Some evidence suggests that, like the Khajiit, the Argonian lifestyle is tribal. However, considering references to the Royal court of Argonia and the existence of the Shadowscales, the presence of some form of organized Argonian government is indeed possible. 
In addition to the reptilian Argonians, who are today Black Marsh's most visible denizen, there were once tribes of men, the Kothringi, Orma, Yespest, Horwalli - and tribes of mer - the Barsaebic Ayleids and the Cantemiric Velothi - and even a tribe who may have been related to the Khajiit of Elsweyr, the vulpine Lilmothiit. Some were sent to Black Marsh as refugees or prisoners, other settled along the coastal waterways and adapted to its strange and usually insalubrious environment. These are thought to have died out due to the Khahaten Flu. 
There is also a breed of Argonian called the Naga. They generally lived in the inner swamps of Black Marsh and had very little contact with the Imperials and other races that were not native to Black Marsh.
- The Elder Scrolls: Arena
- The Elder Scrolls II: Daggerfall (mentioned only)
- The Elder Scrolls III: Morrowind (mentioned only)
- The Elder Scrolls Travels: Stormhold
- The Elder Scrolls IV: Oblivion (mentioned only)
- The Elder Scrolls V: Skyrim (mentioned only)
- The Elder Scrolls Online
- ↑ Provinces of Tamriel
- ↑ The Elder Scrolls III: Morrowind
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 An Elder Scrolls Novel: The Infernal City
- ↑ 4.0 4.1 Skyrim's Rule
- ↑ 5.00 5.01 5.02 5.03 5.04 5.05 5.06 5.07 5.08 5.09 5.10 5.11 5.12 5.13 5.14 Pocket Guide to the Empire, Third Edition: Black Marsh
- ↑ 6.0 6.1 The Elder Scrolls IV: Oblivion
- ↑ Ordo Legionis
- ↑ The Armorer's Challenge
- ↑ 9.0 9.1 The Argonian Account, Book I
- ↑ 10.0 10.1 Pocket Guide to the Empire, First Edition: Wild Regions
- ↑ A Short History of Morrowind
- ↑ 12.0 12.1 12.2 12.3 12.4 12.5 12.6 The Elder Scrolls: Arena
- ↑ Cicero's Journal, Volume I
- ↑ 14.0 14.1 http://www.imperial-library.info/content/infernal-city
- ↑ Gameplay from The Elder Scrolls IV: Oblivion
- ↑ The Elder Scrolls V: Skyrim - dialogue with Talen-Jei.
- ↑ Argonian Ceremony
- ↑ The Argonian Account, Book II