The Underdark is filled with dozens of lakes, rivers, and even entire seas, which in turn are filled with numerous aquatic species that have never glimpsed the harsh light of the sun. Kuo-toas, rogue bands of sahuagin, lost tribes of Tritons, and other strange and exotic races swim the blackened waters. The drow despise any challenge to their rule below the earth and view these aquatic races as adversaries to be decimated.
Long ago, a small city of drow began to prosper along the banks of an immense underground lake. However, the lake also hosted a substantial kuo-toan settlement. Clashes between the drow and kuo-toas were common, with the kuo-toas raiding the fragile ships that the drow were using to cross the lake. The attacks became more bold when the kuo-toas realized that without crossing the lake, the drow were trapped - there were no other tunnels leading out of the city.
Salvation for the drow came in the form of a young alchemist who created a vile concoction, which he dubbed blackwater. When dumped into water, blackwater immediately eliminates the oxygen, suffocating most aquatic beings that are caught in its area. The drow "navy" commissioned the alchemist to create as much of the substance as possible. Barrel after barrel of blackwater was dumped into the lake, which had the desired effect of eliminating any kuo-toas that dared to venture close to the surface. Unfortunately, the poison also eliminated most of the fish, crustaceans, and other wildlife that the city was dependent on for food, For a few desperate months, the drow clung to survival while ships went to other cities and bartered for food.
For his efforts, the alchemist was murdered in his sleep to ensure that the knowledge of blackwater would not spread beyond the city. However, this plan failed, and soon other drow became aware of the potent concoction.