From the description:
Long ago, before the oceans receded, a great temple was built to worship a creature of immense power, an aboleth. When the oceans receded, the creature and many of its servants were trapped in the depths of the temple, unable to escape back into the open water. In the centuries since, a group of explorers located the top of the temple. Appearing to be nothing more than a ruin on a small island, it is in fact the entry point to a vast underground and underwater complex. As they explored further, many fell under the sway of the aboleth. Others were warped by the alien energies that filled the place. Now, the top of the temple is dominated by sahuagin and their chuul who worship the strange alien obelisk that can be found there. Beneath this, the explorers, driven mad and warped beyond recognition worship at the water’s edge, seeking ways to get closer to their aberrant master. Deeper still beneath the water lies the long forgotten temple itself, and inside, the aboleth and the treasure it guards, the Crown of the Deep.