Castle Ravenloft

From 1d4chan

Castle Ravenloft is the ancestral home of Count Strahd von Zarovich, the vampire darklord of Barovia. This makes it the centerpiece of every single iteration of the "battle Strahd!" adventure module that has made its appearance in literally every single edition of Dungeons & Dragons, and as such the entire Ravenloft setting was literally named in its honor.

As a result, "Castle Ravenloft" is often referred to as one of D&D's most iconic dungeons, and the speaker could be referring to any of the following modules: