"It has been called the Garbage Heap of Tamriel, to where everything rotten and despoiled eventually flows. Its borderlands and coasts have been ravaged by civilization after civilization, but its heart is inviolate, for so poisonous is its air, ground, and water, its mysteries are secure."
"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 was clear. The plague began in Stormhold in 2E 560, and quickly spread to every corner of Black Marsh, killing all those not reptilian stock. For over forty years, it held the Province in its grip, decimating entire cultures (notably, the Kothringi) and driving outsiders from the land."
"Even Tiber Septim, it was said, thought twice before conquering Black Marsh for his new Empire."
http://www.imperial-library.info/content/pocket-guide-empire-third-edition-war-trees-argonia-and-black-marsh