Once every decade, the tides of the Empyrean Ocean recede far enough to reveal the highest eaves of a mysterious undersea tower. Long ago this was the eldritch fastness of a powerful mage, but now the tower is known simply as the final resting place of the fabled Black Pearl -- an artifact rumored to bring doom upon all who dare to posses it. Tonight the moon nearly fills the sky, and the tides have already begun to recede. The PCs have 8 short hours to explore the tower before the dark waters return.