In 1987, Universal Pictures released a new John Carpenter film entitled Prince of Darkness. The film starred... you guessed it, Donald Pleasance. And Pleasance played a character named... Father Loomis -- a priest who must help stop the devil from ending the world, through, of course, a vat of green ooze. Sound crazy enough?
The similarities of this film to the overall strange nature of Halloween III suggest that Carpenter most likely envisioned this tale as the next entry in what he envisioned as the future of the Halloween franchise. Remember everything you've heard about his desire to release a new Halloween-themed film each year, with each one unrelated to the previous and not featuring Michael Myers? And remember the fact that no one seemd to like H3 and so this idea was scrapped? Well there is bountiful evidence to suggest that this story would have been a prime candidate for the continuation of this aborted idea.
The story was written by Carpenter, although credited to the fictitious Martin Quartermass. It is strange enough to be similar to the oddity of H3. And perhaps the most blatant piece of evidence... the main character is named Father Loomis. (And of course, he's played by Donald Pleasance.)
Another less interesting piece of evidence is that Universal is the distributor. Starting with Halloween II, they bought into the franchise and would presumably have distributed future entries if H3 had been successful. The fact that Carpenter was able to corral them into releasing this film, which bears similarities to H3 and the feel of the Halloween series in general, suggests that J.C. had the last laugh indeed.
Whether or not this would have been a good way to continue the series is of course debatable. But I think it's better than any of the actual sequels. Sure, Halloween 4 had its moments, but everything after that was dismal when compared to Prince of Darkness. An unfair comparison, you might say. But I think I have shown that it is fair, given the similarities of this film to the way Carpenter wanted to continue the series. When you see this film in the video store, imagine the word "Halloween" above it. (Halloween: Prince of Darkness.)
And thus this film remains, in my mind, the "lost entry" in the Halloween franchise.