During my early years of playing D&D (focused around the Rules Cyclopedia), I remember I had a set of adventures (I think it was 3?) that came in a boxed set. They could be run separately, or the maps could link up for an epic undead hunting event.
Does this sound familiar to anyone else? I can't remember the name, and I've always regretted not putting my friends through their paces in there (I found the whole thing a little intimidating at the time).