Between the Valley of Damara and the shores of the Moonsea lies the newly formed Marrow Vale. Decades ago this land was covered by a great glacier called the Glacier of the White Worm which slowly melted to expose the bones of the mountains on which the glacier once stood. Whatever the cause of this climate change the ensuing floods destroyed most of the regional life and destroyed cities and towns in the region.
Foremost amongst the destroyed cities was Mulmaster, the corrupt center of Zhentarim trade and the worship of Bane which was completely immersed under the swelled currents of the Moonsea. Though in the past decade, the sea has slowly receded and the skeleton of the great city waits again to start over.
After that, take a look at your wiki. There is some more helpful info there.