TedStixon

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.

TedStixon

Answer: I read the movie tie in book after the first Halloween came out. As I remember it, a boy with a crippled leg was in love with his own sister. During the Samhain (harvest end/halloween) festival, he killed his sister and her soon to be husband with a knife. The tribe tore him apart, and the chieftain cursed his soul to forever repeat his crime forever, etc. The book or following books in the series described Michael Myers' grandfather or great-grandfather losing his mind at a dance and killing a dancing couple. Every other generation or so the soul takes over a decendant, leaving enough people alive to continue the curse.