The Marble Sanctum

From the introduction:

Thousands of years ago, a powerful necromantic sect forged the Marble Sanctum from within the limestone bedrock of a remote tropical island. Using a magical artifact called the Tao Engine, they pressurized and heated the limestone in place to create a massive marbleized network of rooms and passages. It would become a fortress and center of magical research for the faction’s leading arch lich-mage. It would also become his eternal tomb in the last battle of the Azi-Schism.