Long ago, man built an amazing civilization, the one Plato recalled as Atlantis, and ancient Mesoamericans as Mu. This civilization created fantastic developments, and knew the ways of magic well. Their great cities dotted the landscape, and they had mastery over the world.
But they were full of hubris, and wrought their own demise as a result of certain of their own great works. A cataclysm occurred, and the entire civilization sank beneath the waves. It did not, however, perish. Instead this sinking was magical, and separated these now “sunken” lands from those we know. The sky above was replaced by a vault of water, and everything was deluged.
The civilization did not escape unscathed; in fact it was effectively destroyed by this event. Monstrous beasts, once held in check by the powers of this once-great civilization, now roamed the lands accosting the people freely. The people of the sunken realm had to hide in caves or whatever they could find, and eked out a very meagre living for centuries. Somehow small groups of people here and there managed to survive.
Now civilization has emerged once again in the sunken lands, though only barely. People have emerged from the caves to rediscover their world. Some tribes of men have formed new cities on the rubble of the ancient ones of their forebears. Others have remembered the secrets of taming the monsters. Yet others are busy rediscovering various forms of magic, and the gods that their ancestors worshipped. A few have even discovered that there are other worlds that can be visited by means of gates, including the Underworld, the Water Vault, and others.
Your organization is a small settlement, band, or a cult, or the like, seeking to grow it's power and presence in this sunken realm.