As told by the Madu trader, Vati to the adventurers helping to get him home:
Once we were a great people. Devote worshippers of Oghma, the Binder of What is Known, we collected knowledge from all points of the world and brought it to benefit us. We built dams to direct the water and protect our crops. We build many libraries and universities for study. Our cities gleamed with tall buildings of white marble. The people suffered little from disease and illness and those who died before their time were often brought back to life.
All was not perfect, of course. Those capable of study were admired and given all the opportunities of our society. Those who lacked the faculties or the discipline were treated well, but they were still our servants. Villages and cities tended to spring up around the illustrious halls of learning, while other settlements held little interest for us. Those who opposed us were dealt with harshly by our clerics and holy warriors, perhaps too harshly at times, but the fall of our civilization would have meant a descent into the war and madness of the past.
Not all knowledge was permitted for study, which is the origin of how our civilization eventually fell. A secret sect emerged within our tradition called The Black Water. They studied fell magic, such as kidnapping and ransoming souls, contact with the Far Realm, illusion and trickery, and necromancy and planar manipulation. We stomped out Black Water adherents without mercy when they were found, but the knowledge was intoxicating and many young people fell under the sway of the Black Water, drowning in its depths.
One of these young people was The Whispered One, then known as Vecna. Vecna was a brilliant cleric of Oghma and gifted healer. His desire for knowledge of the human body was complete in our tradition, but he wanted more. He was recruited by The Black Water and was secretly allowed to carry out gruesome experiments on those in our prisons. Vecna did not stop there and his thirst for forbidden knowledge knew no bounds.
Eventually Vecna was discovered. Rather than go quietly, Vecna used his friends in the prison guard to stage an uprising, including the warden, an upright knight named Kas who had also been seduced by the secret knowledge and its resulting power. It is true, also, that we were guilty of much hypocrisy, as you might imagine when one has absolute power. The forbidden knowledge they sought at first was simply truth, held from them by our order, but the line between hidden knowledge and dangerous knowledge is a hard one to define.
Vecna and Kas started a war that lasted two hundred years, proving by their unnatural longevity that they were dabbling with dark powers. Our dams were destroyed, our lands flooded and most of our cities leveled. We were clearly losing this war and we were forced to take shelter in the marshes, the wreckage of our society. With the Oghmaites essentially defeated, Vecna and his people ruled for hundreds of years until in-fighting destroyed them. Kas betrayed Vecna, as their kind was wont to do. Vecna was said to have ascended to godhood, while Kas was eventually defeated by other Black Water Vecnites. Petty rulers have watched over this land and hunted us down for many hundreds of years since then.
Our prophecies say that an ancient artifact lies in the Grand Temple of The Whispered One, one of the many ziggurats that dot our land that were once great libraries and places of learning. If this artifact can be brought to bear against our enemies, our prophesies say that Oghmaites may restore our lost kingdom, and with proper humility, rule for thousands of years.