The tower itself is the ruined remains of one of Daigon's old fortresses, and legend has it that the large lake was filled by the tears shed by the order in mourning to the death of their Lord. However more recent cartographic studies have proven the 'sea' is linked with the Cursed Sea, explaining the saltwater for which the lake is named.
The Fallen Keep Beneath the island lies a great aquifer, filtered through the porous rock of the island itself which filters out all that is harmful and leaving only pure, sweet tasting water behind. The Sisters encourage supplicants to the order to drink freely from the waters, claiming that they bring those who taste it closer to Daigon.
In truth while the porous rocks do indeed filter almost all the toxins from the waters of the Saltwater Sea, they also instil in the waters an unknown chemical compound which acts as an hallucinogenic for all the known races of Nagath. Drinking from the aquifer regularly also has a narcotic effect, filling supplicants with euphoria, making them more easily manipulated by the Sisters and also giving the supplicants an addiction with which the inner order gain more control over their followers.