Full spoilers for all three Fear Street movies below! As of right now you can stream all three movies in the Fear Street trilogy on Netflix and with the release of Fear Street Part Three: 1666 we finally have a complete understanding of the Sarah Fier curse on Shadyside in the movies. What we learn about the curse in the first movie of the series (Fear Street Part One: 1994) is best summarized in a town rhyme that read: "She reaches from beyond the grave to make good men her wicked slaves. She'll take your blood. She'll take your head. She'll follow you until you're dead." This becomes literal after several killers go after the protagonists in the film, but the sequel offered a little more context and the third film fully revealed its secrets.
Fear Street Part Two: 1978 revealed a series of tunnels under Camp Nightwing that lead to a wall with the names of all the killers etched into it. Something was causing these people to become possessed by the witch's curse and kill the other people in the town. The biggest development though is that the curse can seemingly be ended by the reunion of Sarah Fier's body and her severed hand, which she is believed to have cut off as an offering to the devil. With the knowledge of where the hand is buried, under the hanging tree inside the Shadyside mall, Deena and Josh retrieve it to take it back to the spot where they discovered Sarah's remains in the first movie. When the hand is placed however it causes Deena to have a flashback, seeing the world through Sara Fier's eyes and setting up the third film.
In Fear Street Part Three: 1666, the cast of both movies return playing the parts of various settlers that live in the town that will one day be split into Shadyside and Sunnyvale. Deena takes on the part of Sarah Fier, a seemingly normal girl and one who, like Deena in the present day, has a secret romance (Sam actress Emily Rudd in the role of Hannah Miller). As the film progresses it becomes clear that while trickery and witchcraft are afoot, Sarah Fier had no hand in it and neither does the actual woman believed to be a witch outside the town. In fact the sequel makes it clear that Sarah and her legacy were set up by paranoid townsfolk, a fellow teen that got turned down (and had his boner mocked), and Solomon Goode, an ancestor of his present day character Sheriff Nick Goode (both played by Ashley Zukerman).
In fact, Fear Street Part Three reveals that the "Sarah Fier" curse is entirely an inaccurate a name anyway as it's actually the Goode family curse. 1666 confirms Solomon Goode made a deal with the devil for prosperity and power and in exchange would offer up a member of the town to commit a massacre. This tradition of offering up innocents and their blood to maintain power continued throughout the generations and it's shown that Nick Goode carried out the ritual in both 1978 as a young man and in 1994 as an adult.
What 1666 also makes clear is that Sarah Fier didn't cut off her own hand to sacrifice it to the devil, Solomon Goode cut it off while attacking her and pleading with her to join him. From there the hanging and death of Sarah Fier takes place and the actual Curse of Sarah Fier shown as she whispers to Solomon Goode that "The truth will come out. The truth shall be your curse. It will follow you for eternity. It will shadow you forever." It's also shown how her body ended up in a grave off the beaten path and nearly unmarked save for her chains, her friends took her body and gave her a proper burial.
Once the truth is discovered it leads into the final act of the film which returns to the present day where the many killers converge on the Shadyside mall while Deena, Josh, Martin, and the grown-up Ziggy try to fend them off and end the curse. In the end it comes down to Deena facing Sheriff Goode in the tunnels. After forcing Goode to make contact with the giant beating heart that births the Shadyside Killers, the Sheriff sees all the victims his family has caused and hears Sarah Fier's final words just before Deena stabs him in the eye and ends it.
But that's not all, as Fear Street Part Three: 1666 comes to a close, the grimoire that Solomon Goode stole from the town witch is seen in the stone room that previously held the names of the Shadyside killers which is buried under the Shadyside mall. As it sits there behind police tape, a pair of hands reach forward and take it, ending the franchise and potentially setting up the next story.
All three Fear Street movies are now streaming on Netflix.