In the 90s, I got a strange hankering to run D&D using the D&D Cyclopedia and setting all my adventures in the Mystara setting. In fact, this was really where my love and awareness of Mystara as a setting really began.
Strangely enough, my campaign -- based in Karameikos -- began to incorporate elements of Ravenloft into it. Or perhaps not so strange. The native Traladarans of Karaemeikos had a culture that echoed Transylvania and nearby lands -- perhaps having the Vistani (a gypsy equivalent in Ravenloft) as a known but feared sight in Karameikos wasn't that far-fetched?
In any case, while still early on in my campaign, I began incorporating modules like RQ1 - Night of the Walking Dead to my players. I remember this being a watershed event for me, because it was one of the times that I was able to, as a GM, lie in character to my players. The secondary villain was able to bluster like an idiot and a fool well enough to misdirect the two biggest fighters away from the party (they had horses) before his ruse was revealed by a sharp-eyed and sharp-witted player. It was a tough battle, but the non-fighters were able to prevail over him, and rejoin the muscle of the party for a showdown with the big bad.
It was also here that I began cultivating a taste for horror in my D&D adventures, and began plundering from Ravenloft and Cthulhu and Chill for horror elements to inject into my own adventures.
I'm not necessarily talking about slavering, red-eyed monsters. I'm talking about the gothic horror feel, about shadowy fields, and mist-choked forests. I'm talking about doors that suddenly nudge open with a creak, or shapes in windows that suggest lurking malevolence. I'm talking about the loss of light in a dark dungeon, as something large takes measured, metal-tinged steps ever closer to your hiding spot.
And when the fight breaks out, players are relieved to be able to finally face something and do something to end that building horror.