X2: Castle Amber
X2: Castle Amber is an adventure module for the original Dungeons & Dragons ruleset, released in 1981 by Tom Moldvay. It was inspired by (and/or an adaptation of) the Averoigne pulp fantasy novels written by Clark Ashton Smith, aka one of the guys whose pulp fantasy tales inspired Gary Gygax to make D&D in the first place, crossing over the Mystara setting with Smith's fictional medieval French province of Averoigne.
The module revolves around Castle Amber, a place that was once home to a family of powerful but insane wizardry-practicing nobles, the Ambers. However, when the patriarch Stephen Amber (Etienne d'Amberville) was betrayed and murdered by his relatives, his ghost placed the castle under the curse. The party takes the role of adventurers traveling through Glantri, only to be pulled into the castle by a mysterious magical mist as night falls, one that quickly kills them if they try to turn away from the castle - yes, this may sound familiar, but this actually predates I6: Ravenloft by a good two years, even if this is where the Mists of Ravenloft obviously have their roots. With no way out, the party is forced into the castle to shelter from the mists.
Once trapped inside Castle Amber (or Château d'Amberville), the party's goal is simple: escape! Evading traps and working around the demented yet powerful members of the d'Amberville family, as well as the other, often hostile, creatures that have been brought here by the mad mages. Ultimately, the party will need to find and open a hidden portal that will allow them to explore the wilderness of Averoigne, where they must evade the mage-hunting Inquisitors and locate the inter-dimensional tomb of Stephen d'Amberville. Only here can they break the curse and return to the relative sanity of Mystara.
Castle Amber was one of the two debut modules for the aranea, lupin and rakasta races, all of which appear as monsters. It was also the birthplace of the Neh-thalggu, a brain-collecting aberration, and the Magen, powerful golem-like constructs that are essentially an advanced form of homunculus imbued with wizard-like spellcasting abilities. There's also a demon, the "Death Demon"; which thread starts getting lost when M1: Into the Maelstrom drops a LOLrog on the party and is wholly abandoned in the Immortals line which decides all BXCMI demons shall be epic herewith.
Reprint and 5e Conversion
Part of their ongoing licensing agreement with Wizards of the Coast for the "Original Adventures Reincarnated" line of books, Goodman Games got permission to reprint and convert the module to 5e as "OAR #5 Castle Amber." It was released in Fall 2020.
Reviews of it are generally positive though people note that most of the essays basically are "READ CLARK ASHTOBN SMITH!" The 5e conversion is pretty much on point though.