After Silas betrayed Rebekah, he developed the cure. Qetsiyah demanded that Silas drink it so that they could spend the rest of their mortal lives together. All the vampires from The Originals and The Vampire Diaries had a keen interest in finding her.
Silas’s search led most of the main characters, including Rebekah, from The Originals, to a remote island off the coast of Nova Scotia, but it was Katherine Pierce who managed to retrieve the cure and free Silas.
Before Klaus died, she offered Rebekah, a character from The Originals, an offer of peace: the cure. She told her sister that the cure was waiting for her in Mystic Falls, and asked Caroline Forbes to get it for her.
The cure was never mentioned in The Originals until the end of the series, despite Rebekah and Klaus’ desire to obtain it. Fans may have wondered how long Rebekah would have to wait to live a human life.
While Klaus’s gesture in The Originals reinforced how much he had changed for the better since his early days in The Vampire Diaries, it also highlighted the complicated nature of the cure and how it worked.
Presumably Caroline will keep his promise, and when Rebekah, a character featured in The Originals from The Vampire Diaries universe, finally dies of old age, that will be the end of the cure.