To put it simply, the Ring was Sauron’s horcrux.
In the Silmarillion, it is pointed out that the world itself was used in the same way by Melkor/Morgoth, the Vala who was the original Dark Lord. Sauron was just his lieutenant.
Morgoth poured so much of his own self into the world in order to control it that he weakened himself enough that he was defeated and expelled from the world. But the evil he put into the world is now forever part of it.