B1: Legacy of the Unknown is third-party sequel to the classic TSR module, B1: In Search of the Unknown. The new adventure sends the party in search of Roghan the Fearless and Zelligar the Unknown, the missing creators of the original dungeon, Quasqueton. From the front cover:

The wizard and warrior disappeared, leaving their vast underground complex behind. Years later a band of adventurers discovered the fortress and caverns and cleared out the evil that had take up residence. But what happened to the mysterious owners and what was their fate. [sic] A single clue has been discovered and it will surely reveal the Legacy of the Unknown!

Legacy of the Unknown is an adventure designed for 4 to 6 characters of 2nd to 4th level. The scenario requires the First Edition Advanced OSRIC^TM rules system.