While browsing through Google, I accidentally discovered the existence of a lost Gegege no Kitaro fan film. There's absolutely no information about this project in English, which is making research difficult. From what I've found, animators from the third and fourth Kitaro animated series allegedly worked on the film. The film is titled "女禍" and was screened privately in 1989, after the third anime finished. Some sources claim to own VHS copies of the film, but none have surfaced online.
Here's an image of the poster for the film. I plan to write about this soon, but I was hoping I could bring more attention to it, considering I haven't found any mention of it in English.