In the history of the hobby, there are a handful of early products that might rightly be called legendary. 1976's Palace of the Vampire Queen, written by Pete and Judy Kerestan, probably qualifies for such status. The first in a series of "Dungeon Masters Kits" published by a company called Wee Warriors, Palace of the Vampire Queen is the first stand-alone adventure module ever produced, which probably explains why its earliest printings were "distributed exclusively by TSR Hobbies, Inc.," as the credits page notes. There was a demand for prefabricated adventures and this adventure was written to meet it.
At the time, the only other adventure available was "The Temple of the Frog" in Supplement II to OD&D and, awesome as it is, it was probably too idiosyncratic to serve as a model for others to use in creating their own adventures. Palace of the Vampire Queen, on the other hand, is a much more "traditional" scenario, combining the classic elements of the post-Dracula vampire myth with D&D distinctives, like demihumans, giant vermin, and magic treasures. In a sense, one could call it an ancestor of the more well-known Ravenloft, except that Palace of the Vampire Queen contains only the thinnest plot, being more of a location-based adventure rather than an occasion for romance novel-level melodrama.
The Palace of the Vampire Queen is thus a very bare bones affair, but that probably makes it more immediately useful. Its basic assumptions are those of most D&D campaigns, namely that the characters are a party of "adventurers" looking to acquire fame and power by venturing into Chaos-tainted locales and battling the monsters that dwell therein. Introductory text explains that the daughter of the dwarf King Arman of Baylor has been kidnapped by the dreaded Vampire Queen, providing some context for the PCs' actions, but there's little else to frame the story nor are there additional rewards for rescuing the unnamed dwarven princess from the Palace.
Instead, what we get is a five-level dungeon intended as "only a basic outline -- you can make it a dramatic adventure." Each of the five levels has two maps each, one that is keyed and one that is not, the latter being available "to speed game play" if the referee prefers not to have the players map the Palace themselves. The maps are unusual in several respects. First, they're not presented on a grid, instead using a scale of one-quarter inches equaling six feet. [This is in error; please forgive my aging eyes --JDM] Second, they use non-standard symbols, eschewing those presented in the LBBs in favor of its own. Finally, the maps have rather attractive decorated borders as you can see below.
The same level of attractiveness applies to the map keys, which, despite their oddities, are clear and easy to use, far more so than the convoluted keys of "The Temple of the Frog." In this way, Palace of the Vampire Queen established a standard of presentation that, while not wholly adopted by others, would nevertheless exert an influence.
This was a product intended to be picked up and used by any referee, regardless of the campaign he was running or the characters being used in it. That alone makes it remarkable, even if by the standards of TSR and Judges Guild adventures from just a short time later, it feels very "flat." Palace of the Vampire Queen is a milestone in the history of the hobby and it certainly deserves to be more well-known than it is.