There are two versions of this movie: the original (1971) and a 1983 version ("The Witching"), the latter of which is not worth watching. If you stick to the original, you will find a flawed, but surprisingly effective and dark, horror movie.
While some of the writing and editing is a bit heavy-handed and, frankly, clumsy at points, Pamela Franklin manages to carry this movie admirably. Frankly, the director could have cast just about anyone in the other parts, and I'm not certain it would have mattered. She is at all points the focal point of this film. As such, Welles is just an audience draw. He was by no means essential.
In terms of subject matter, I would put it in the class of movies that came out immediately after "Rosemary's Baby." It is not that movie's equal by any standard. (Stylistically, it is perhaps closer to Romero's "Season of the Witch.") There are many of the same themes: child-birth used for "unnatural" or potentially evil ends; secretive societies within "normal" settings; an alternately naive, sweet/doe-eyed female who is also intelligent, strong-willed and quite perceptive; the often "absent" and not entirely trustworthy husband.
In the end, while this film is not nearly as focused and sharp as RB, the choppy, low-budget feel to the camera work and editing in Necromancy actually works to keep it somewhat phantasmagoric and nightmarish. The ending sequences, with the twist, are actually quite good.
As an aside, the film does appear to have been filmed in part in Los Gatos, California. The funeral scenes appear to have been shot near the Lexington Reservoir, and the exterior shots are likely of houses near the downtown that still stand unchanged, as Los Gatos has retained its "preserve the past" zoning standards --a not unimportant point given the theme of the movie.
