I could be wrong, but I don't think they started referring to the Devil or Prince of Darkness/Fire on the series until Cassandra showed up in the Spring of 1968. While Cass is incanting over her favorite Dr. Lang doll/pincushion, at one point she exclaims "my powers were given to me by the Devil himself!" If you go back and watch the original 1795 episodes, Angelique's original spells are all more like hedge witchcraft, involving herbs, object links belonging to the person, etc. Demonology enters the picture when she creates the bat to be her servitor and spy on Barnabas but even then I don't recall her mentioning the Devil, though Hell may have been mentioned.
I could never understand why they started up with the Satan stuff in 1968 but then it hit me: Rosemary's Baby was big news so the writers decided to introduce that element.
I guess that even though the Diabolos sequences come across today as atmospheric but on the edge of a little TOO extreme (GREAT voicework from the guy who played Diabolos), the production unit never again revisited this particular terrain. But it was enshrined forever in our memories because THESE were the episodes chosen for the series' Viewmaster reel.
In the booklet that came with the reel, incidentally, "Diabolos" is referred to as Balberith. I don't know whether that came from a DS script, or if whoever wrote the booklet came up with that name themselves. There's a Wikipedia entry on Baal-berith (who, like many demons, started out life as a Canaanite Deity) but I quite like this site where he is described as "chief secretary of Hell"... which fits rather well with the DS representation of him, don't you think?
http://www.whiterosesgarden.com/Nature_of_Evil/Demons/List_of_Demons/B_contents/baalberith.htmcheers, Gothique (rhymes with "eek!")