The Ladies of the Lake are a pagan sisterhood of priestess/enchantresses. They live apart from humanity and instruct women in the spiritual and magical arts.
As an organization, they certainly have an agenda, though none but they knows for certain what it is. In the old days, they were at different times rivals and then allies to the druids. Now they are one of the few pagan organizations that still wield significant power.
Their magic stems from the powers of lakes and water. It is said they can create magical homes underwater in the same way that the old pagan “land of the dead” was under water. This is generally done beneath an existing body, though it seems that the most powerful among them can create new bodies of water as well.