The Black Captain's Tower
The Black Captain’s Tower was raised overnight hundreds of years ago by the undead reaver known as The King of Blue Graves, possibly as a staging ground for his undead army before invading Mazara, or perhaps intended as his throne once he had captured or slain the noblewomen responsible for his demise.
The tower itself long ago fell to the elements, assisted by Mazara’s army after the Captain was defeated, but it’s dungeons reached well below the water line and so remained unexplored for generations.
A rare conjunction of the moons recently caused extreme spring tides, though, creating a window of several hours each day in which explorers could safely enter the complex. Or not so safely, as it turned out. After fighting past a band of kobold bandits who had used the dry upper levels as their base, a brave band of adventurers almost met their end when they descended deeper and found a band of sahuagin performing a human sacrifice to raise the Black Captain and were betrayed by one of their own. The specter of the Black Captain chased the survivors from the dungeon, and no one has dared venture into the ruin since.