Thorn refers to an ancient, dark and supernatural demon that bestows great power upon someone who was possessed by it, dating back to the time of the Druids.
The Cult of Thorn was a cult of druids whose activity spans over several millennia. The nature of the cult is different, depending on which version seen of Halloween: The Curse of Michael Myers.
In order to save themselves from a demon that spread sickness and death, the cult places a curse on a child from their tribe, currently Michael Myers. The child is then possessed by Thorn, which gives the child superhuman abilities, and causes them to kill their entire family on the night of Samhain (Halloween), whenever the Thorn constellation appears. They planned to place the curse on Danny Strode after Michael made his final sacrifice.
The primary antagonists are the doctors of Smith's Grove, who seem to be conspiring with the Druids to use the Thorn host's cursed DNA for scientific experiments because Terence Wynn, the mediator between the cult and his staff, wants to control the power of pure evil. Steven is implied to be the result of Wynn cloning Michael using Jamie as a surrogate, and the cult presumably kidnapped Danny and Kara to clone another baby. However, exploiting Thorn for personal gain seemed to cause the cult to lose control over Michael, who ultimately turned on the doctors and killed them all.
The cult aids Thorn as it forces its host to kill their family members. They also seem to care for him while he's in his resting periods. The Man in Black leads the cult members and takes orders from Thorn. Dr. Wynn, the Man in Black during Michael's possession, and several other cult members, decided to try and harness the supernatural abilities that Thorn possesses.
The Curse of Thorn is often associated with a constellation of stars that could appear during Halloween. In the time of the Druids, to prevent mass death among the tribes, one child from each tribe was chosen to be possessed by Thorn. It would then sacrificially murder its host's entire family on the night of Samhain (All Hallow's Eve), which in turn it would promise to spare their community from disastrous events, such as plague and drought. This possession also appears to give the cursed subject inhuman strength, durability and immunity from death.
- According to Tommy Doyle in the Producer's Cut, out of all the ancient runes, Thorn had the most negative influence.