Trivia: The production of the film was notoriously troubled. There were numerous writers working on numerous drafts of the script before one was finally settled on. Filming was hectic with many scenes being omitted or re-written on-set. And after the first cut was complete, a poor test-screening lead to the film going through drastic reshoots that altered much of the story, including the addition of a brand new ending, which had to be written around the fact that co-star Donald Pleasance passed away shortly after the end of principal photography.
Trivia: Originally in the early 90's, Quentin Tarantino was approached to write the film based on scripts he had written including "True Romance." A deal wasn't finished at the time and Tarantino backed away. A few years later, after Tarantino became a successful director, he was again approached about the project. He was planning to produce the film with his friend Scott Spiegel (co-writer of "Evil Dead II") directing, but again, a deal was unable to be reached. According to Tarantino, his version of "Halloween 6" would have taken a step back after the events of "Halloween 4" and "Halloween 5" and told a simpler, stripped down story more akin to the original than any of the prior sequels.
Trivia: The original version of the film that was created before the plethora of reshoots (known as the "Producer's Cut"), was a famous and infamous "legend" among fans. For many years, it was only available illegally and in generally poor condition through bootleg copies that were sold at conventions and festivals, although it eventually found its way online. It wasn't until 19 years after the theatrical cut debuted that the Producer's Cut was given any sort-of official release, when it was included in a Limited Edition Blu-Ray boxed set of the series. It has since also been given a stand-alone release on Blu-Ray.
Trivia: The film went through several titles during production, and the final title wasn't settled on until shortly before the film was released. An early trailer even referred to the film by an alternative title: "Halloween 666: The Origin of Michael Myers", which itself was a combination of titles from two different drafts of the script.
Trivia: Danielle Harris refused to reprise her role as Jamie Lloyd as, among other things, she found the amount they were willing to pay her was insulting.
Answer: The other answer is based on a novel and not on the film itself. According to this film (and the alternative Producer's Cut), the curse of Thorn is an ancient druid curse that must be fulfilled - a chosen one is picked from every "tribe", and made into an almost superhuman figure. This chosen one will then be forever compelled to sacrifice and kill-off his or her own family on the night of Samhain (aka, Halloween night) every year until the curse is either stopped or passed onto another.