Published by Wizards of the Coast.
Let me state for the record that I absolutely hate the title of this module, which I think implicitly places far too much emphasis not just on the character of Mordenkainen but also on the adventure he and his companions had there in 1973. There are few people for whom the prehistory of Dungeons & Dragons is as important as it is for me. Had I the money and time to do so, I’d certainly be flying all over the country, interviewing people associated with the early days of the hobby in order to record their reminiscences for posterity. We’ve already lost far too many of our founding fathers as it is — and their memories along with them. Their experiences at the start of it all are important and ought to be preserved.
Read the Full Retrospective/Review at Grognardia.