Gothick is referring to what is often called the Dark Shadows story "bible," (my memory is failing me, but I think it
is called "Shadows on the Wall").
However, there is no setup for the Barnabas character or hint for what is to come with mausoleums, chained coffins, cousins from England, etc. Not even a hint of the Laura Collins as phoenix character (she's said to be in an institution, I believe, in the character sketch), which was when DS took its first prolonged foray into the supernatural.
"Shadows on the Wall" apparently grew out of Mr. Wallace's original script or notes for "The House," an older television drama presentation, which is going to be aired at the DS Festival this year (wish I could be there
)
This preliminary story bible for DS sets up the Victoria Winters mystery, the Roger as alchoholic scenario (if memory serves correct -- I know he's killed off early on), Jason's blackmailing of Liz, and the very early story lines which were subsequently altered a great deal once the show got underway. It is basically melodrama, with some thematic inspiration reaching back to Hawthorne's "House of the Seven Gables." Nothing supernatural.
Since Wallace *apparently* had left the show before anything supernatural happened, I would question the idea that he suggested, planned, or outlined the Barnabas storyline, although it would be interesting to see evidence to the contrary.
I don't have access to my Program Guide, Shadows on the Wall, etc., but someone could check for sure when he was no longer credited with scripts, and I think it was quite a bit before the first tantalizing appearance of Josette's ghost at the Old House (which preceded the Laura story). I do remember reading that he was still in the background supervising scripts by new writers, but wish we knew for sure how long his association with the show continued.