This is the episode in which Dark Shadows gets right to the heart of things: Diabolos. It sort of amazes me that the series went on for over two years after that, because after you've seen the Devil in person, what else is there?
And the Devil turns out to be quite a reasonable old gentleman, pointing out that Angelique's complaints of Nicholas's vindictiveness and unreasonableness have no basis in fact. I laughed out loud when she said "vindictive". We're talking about the promulgation of evil here, and she expects the Devil to frown on vindictiveness?
But maybe Nicholas had indeed cheated the Devil. The Devil expected that the Super Race that Nicholas was arranging (I can't say creating, since that was Julia's job) would obey only him - i.e. the Devil. If the Devil thought that, then clearly he had never met either Adam or Eve. And yet, considering Eve's origins, he must have been acquainted with her. I guess the Devil was not such a reasonable old gentleman after all.
Incidentally, what a nerve Angelique has, summoning the Devil!