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.
It's most famous feature, the ring of lethally poisonous mists which forces you to explore the castle or die, was actually stolen from the earlier module X2: Castle Amber, as was the idea of magical mists plucking the PCs up from wherever the fuck they were and dropping them right in the adventure - both of which went on to become integral to the Ravenloft setting itself.
It also attempted to integrate replayability by using a random-generation system for several important plot beats and the location of various useful items around Barovia, a system later versions often implemented. While the Gothic atmosphere is thick and heady, and Strahd is a rightly-famous and fascinating villain, the dungeon crawl within the castle itself is a deadly combat-focused meatgrinder.