In the series finale of The Originals, Klaus Mikaelson offered his sister Rebekah what she wanted most in the world: the cure for immortality. The Vampire Diaries introduced the cure during season 4. When a member of The Five arrived in Mystic Falls, the story behind the cure slowly unfolded. The witch Qetsiyah created the immortality spell, cementing their love as eternal. After Silas betrayed her, she developed the cure. Qetsiyah demanded Silas drink it so that they could spend the rest of their mortal lives together. When he refused, she locked him in a tomb with the cure. Every vampire in Mystic Falls had a vested interest in finding the tomb, Silas, and the cure.
The search for Silas led most of the main characters, including Rebekah, to a remote island off the coast of Nova Scotia, but it was Katherine Pierce who succeeded in retrieving the cure and freeing Silas. Bonnie Bennett eventually defeated the ancient witch, encasing him in stone (he escaped following Bonnie's death). Damon Salvatore gave the cure to Elena Gilbert, who shoved it down Katherine's throat. Silas drained it from Katherine, and when Stefan Salvatore killed Silas, the cure went with him. However, when Bonnie returned from the 1994 Prison World (an alternate dimension where the cure still existed) during season 6, she brought the cure back with her. History repeated itself when Damon once again gave it to Elena. Bonnie injected Stefan with Elena's blood during season 8, making him human, and in the series finale, Stefan passed the cure on to Damon.
Before Klaus died, he offered Rebekah a peace offering: the cure. He told his sister the cure was waiting for her in Mystic Falls, and he asked Caroline Forbes to get it for her, saying, "In a handful of decades, when it's time, you can go to Mystic Falls and get it. Then if you no longer wish to be a vampire, it's your choice." It's unclear how long Klaus had known there was a second cure, since Damon and Stefan were initially determined to keep its existence a secret fearing for Elena's safety. The cure was never mentioned throughout The Originals until the series finale, despite both Rebekah's and Klaus' desire to procure it on The Vampire Diaries.
Rebekah agreed to marry Marcel Gerard as long as he accepted she would grow old and die, although this meant her lifelong wish of having children would go unfulfilled. Fans may have wondered just how long Rebekah would have to wait to get to live a human life. The show's creator, Julie Plec, told TVLine how far in the future Rebekah's dream of mortality would come to fruition:
“Damon Salvatore needs to grow old and have babies and grand babies and great grand babies with Elena Gilbert-Salvatore. At the end of their long Notebook life, when they die side by side, when they’re ready to go, he’ll hand the cure over to Rebekah. When you take the cure out of a former vampire who’s taken the cure, they age [rapidly] and die. Basically, when Damon’s ready to go — which is when Elena is ready to go — that’s when Rebekah will get the cure.”
While Klaus' gesture reinforced how much he had changed for the better since his early days on The Vampire Diaries, it also highlighted the complicated nature of the cure and how it worked. The Vampire Diaries season 4 established the significance of there only being one cure, but conveniently it could be passed down from Elena since she had only been a vampire for a short time. Initially, for the cure to work, all the blood had to be drained from whoever had consumed it. Katherine survived the process (only to die later anyway), and it only took a small amount to rid both Stefan and Damon of their immortality, creating another plot hole. Presumably Caroline will make good on her promise, and when Rebekah finally dies of old age, that will be the end of the cure.