From the introduction:
A large book, known as Slerotin's Manifesto, had been venerated within the Onnwal temple of Wee Jas. During the Scarlet Brotherhood's occupation of Onnwal they obtained this prize. After recovery of the troll-skinned book, it was studied carefully by the Brotherhood in the closed city of Heseul Ilshar.
The book was found to contain many powerful spells, but more important to the Brotherhood were the history and details of the ancient Suel Empire. After reading two sections that told of an ancient relic "The Darkness That Holds All Shadows" and a powerful magical portal "The Null", the Brotherhood decided to send an expedition into the Sea of Dust.
The Brotherhood's expeditions were costly in terms of loss of life. The Yeomanry spies who closely watched the Brotherhood's local activities reported what little they could determine about these forays. While the Yeomanry does not know the details of their plans, they are correct to be worried. The Brotherhood plans to travel back in time using the Null, and once there steal the cubic relic from Xodast's treasure vault.
For 5th - 8th level characters. A high-resolution map ("The Ancient Suel Basin") also was provided as a separate 1-page PDF file.