Spoiler warning: Spoilers for the complete film or season named below.
Rachel dies and returns as the new Witness, obliged to attend future weddings in Nicky’s bloodline. Something Very Bad Is Going to Happen ends the relationship without freeing her from every consequence of the wedding.
What makes the curse spread?
Rachel must marry someone she believes is her soulmate before sundown. Nicky refuses at the altar. When the deadline passes without the marriage, the curse reaches his family.
He then rushes to complete the ceremony, but Rachel no longer believes he is her soulmate. She has lost that conviction because he dismissed the danger she tried to explain. Completing the words cannot restore a belief that has broken.
The sequence matters: failing to marry in time spreads the curse; marrying without the required belief kills Rachel. Combining those events into one vague rule makes the final transformation harder to understand.
Why do some guests survive?
The curse does not kill everyone at the reception. It affects members of the bloodline who are married and do not believe their spouse is their soulmate. Those outside the bloodline and unmarried relatives are in different positions.
Nicky, Boris and Jules survive under the stated rules, while Victoria and Portia die. Nell is not part of that bloodline, and the unmarried Jude carries the curse without facing the same immediate outcome.
This is why an apparently troubled relationship can survive the wedding while another does not. The mechanism concerns the person’s belief, not an external ranking of how harmonious the couple appears.
Why does Rachel become the Witness?
Her spreading of the curse creates the obligation. The previous Witness is released, and Rachel must observe the weddings of the newly cursed family. Creator Haley Z. Boston explains that transfer in the official finale interview.
The character introduction places Rachel inside a family celebration that repeatedly ignores her fear. Her last departure reverses that social arrangement: she no longer waits for Nicky to understand before leaving him.
Yet leaving the relationship and escaping the mythology are separate outcomes. Her new life still contains rules, and the curse remains relevant if she chooses marriage again.
The last goodbye is consequently practical and exhausted rather than a reunion. Rachel has stopped trying to make this particular relationship fulfill the condition on which her safety depended. She walks away with a future, but also with work the failed wedding has imposed on her.
Sources: official ending breakdown; official interview or additional context.