Ravenloft - 1st Edition AD&D
By far my favorite module ever written for D&D, Ravenloft tells the story of the vampire Strahd von Zarovich, an immortal who still pines for his lost love. Set in the land of Barovia, players must not only investigate the strange land, but they must find the weapons they will need to help them defeat Strahd. Strahd is an intelligent foe, and his motivations (and the locations of the tools to defeat him) are randomly selected each time you play the adventure. Since it launch in 1983, it has spawned sequels, adaptations, and a full-on campaign setting. Wizards of the Coast released a 2006 mini-campaign titled Expedition to Castle Ravenloft that was based on the material from the original module, and also a 2010 board game titled Castle Ravenloft.