The Sunken Temple of Arn

From the introduction:

In ancient days the monastic assassins known as the Sons of Arn built a great temple to their strange god on an island off the southern coast.

For many years they managed a troubled coexistence with the other religions. But when they dared to strike down the high priest of the sea god, their island temple was cast beneath the waves forever in an act of divine retribution.

Their knowledge lost and their religion destroyed, the Sons of Arn vanished from the world, leaving only half-remembered whispers in their stead. But now rumors abound of a shipwreck whose lone survivor escaped from a Sahuagin band that makes its home in a submerged ruin eerily reminiscent of the legendary Sunken Temple of Arn.