The Masque of the Red Death gave me nightmares

Dungeons and Dragons has had a wide variety of official settings, which only makes sense for how long it's been around. Some of those settings have really made some extreme changes in the basic concepts of Dungeons and Dragons. In my experience, the one that made the biggest changes was Masque of the Red Death.

Red Death was a subsection of another setting, Ravenloft, the world of Gothic horror. Ravenloft, all by itself, tends to be deadlier than your general Dungeons and Dragons experience. Magic, particularly happy divine healing magic, tends to be weaker.

Red Death moves the play out of out-and-out fantasy and to a Gothic Earth in the 1890s. You know, just like a regular historical Earth, other than some eldritch miasma that causes horrible abominations to arise.

On the one hand, this more mundane setting makes magic even more limited than Ravenloft. Magic items are scarce to the point of non-existence. There really isn't any armor to speak of. 

On the other hand, there are guns. Which might be ineffective against some monsters and opponents with opposable thumbs have access to guns too. At least there aren't misfire rules. 

Still, if a player has to choose between a gun can take a long time to reload and a fireball, they'll pick a fireball every time. Or, heck, even a magic missile.

Really, in many ways, Red Death had a similar feel to Call of Cthulhu. At least in the games I was in, player mortality was really high and the  sense of vulnerability was also very high. 

That said, Call of Cthulhu focuses on cosmic horrors, vast and uncaring nightmares that don't care if you live in madness or die in agony. Red Death is gothic horror, where the nightmares are more intimate and do care about your suffering. It's personal choice which is worse.

From what I can tell, Masque of the Red Death never really took off. There was practically no supplemental material for it. Outside of the folks who were in the campaign that I was in, I've never heard anyone talk about.

It may have been too niche. In many levels, in theme and setting and concept, it went out of Dungeons and Dragons' comfort zone. Even in more bizarre settings, like Spelljammer, it's still heroic fantasy. Red Death isn't just horror. It's horror where the characters are significantly weaker than any other flavor of Dungeons and Dragons.

At the same time, many years and several editions later, my time with the Red Death still stays with me. That campaign gave me the worst nightmares I've ever gotten from a role playing game. The Masque of the Red Death was an odd beast but it did its job well.