Mrs. Blankenship: A long, long time ago, it was a night of great power. When the days grew short, the spirits of the dead, returned to their homes to warm themselves by the fire's side. All across the land, huge bonfires were lit. Ohhh, there was a marvelous celebration. People danced, and they played games, and they dressed up in costumes, hoping to ward off the evil spirits. Especially the boogey man.
Dr. Wynn: I'd like you to come back to Smith's Grove.
Doctor Sam Loomis: Dr. Wynn, you know it is not wise to play Halloween pranks.
Dr. Wynn: Spirits and powers of the flame, attend and witness this ritual. Bear our gifts to Thorn. Open us to the path of Darkness. By these runes transform us. Let the hammer descend upon the Chosen One to whom we offer this sacrifice of Innocent Blood. And then Danny, your journey begins. Kill for him.
Doctor Sam Loomis: I knew what he was, but I never knew why.
Sheriff: Go back to your loony bin. I've got enough problems around here without you stirring up ghost stories.
Doctor Sam Loomis: I suppose it was a ghost who did all of this, and a ghost on the radio last night, and a ghost being carried out of here now."
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