I6: Ravenloft
I6: Ravenloft is an adventure module for the 1st edition of Advanced Dungeons & Dragons. Created as a Gothic/Hammer Horror-themed module by Weiss & Hickman, the authors of the Dragonlance modules/setting, it revolves around the party being trapped in the blighted land of Barovia and forced to seek the end of the monstrous aristocratic vampire, Strahd von Zarovich, who rules over his cowering mortal flock from his perch in Castle Ravenloft.
This module is officially considered the 2nd Best Module of All Time in Dragon Magazine, and on top of being the first installment of the Ravenloft setting it has spawned at least one remake in each edition: RM4: House of Strahd in AD&D 2e, Expedition to Castle Ravenloft in 3.5, and Curse of Strahd in 5th edition.