"The Cure"
The cure was created by Qetsiyah, she created it for Silas to take so that he could become a mortal witch again and be stuck on the other side with Qetsiyah, in revenge for betraying her and giving the other immortality serum to Amara.
The cure was buried with Silas but once Elena and the rest of the crew found out the cure was a real thing, they all did whatever they could to find it and eventually they did. Once they found the cure nobody wanted to take it, Stefan and Damon only wanted it for Elena but when they found it, she didn't want it. However, Elena did keep the cure and ended up using it on Katherine Pierce during a fight because Katherine almost killed her. At the end of the show almost everyone ended up taking the cure, the only character that remained a vampire in the end was Caroline Forbes.