On the topic of Leviathan:
I was intrigued recently while watching a video that included a clip from a 1967 underground film about the Church of Satan, "Satanis," to learn that in the liturgy of that church, the Evil One was addressed as Leviathan in their invocations.
Perhaps one of the writers actually saw that film at one of the art film festivals in NYC and that is where this came from?
I've always wondered where they got from the Leviathan of the Bible to the notion of DS of an ancient race that once held mastery over the Earth, nomenclature wise.
As for originality, I think the Dream Curse was fairly original, though you may not care for it very much! I've wondered, though, whether the idea for it may have been suggested by E. F. Benson's wonderful tale, The Room in the Tower.
And Count Petofi is a pretty original "creature" although presumably suggested by tales of such magicians as Cagliostro, the Comte de Saint Germain, and Ally (NOT McBeal) Crowley.
Gothick