Sauron is the Dark Lord defeated in the War of the Ring in J.R.R. Tolkien’s legendarium. He created the One Ring to control the other Rings of Power and used it to dominate much of Middle-earth.
Sauron originally served Morgoth, the first Dark Lord, during the First Age. After Morgoth’s defeat, Sauron became the central enemy of the later ages and established the land of Mordor as his stronghold.
The One Ring’s destruction in Mount Doom ends Sauron’s power because he had placed much of his strength into it. His defeat is therefore tied to the ring’s destruction, not simply to a battlefield victory.
Morgoth is an earlier and greater enemy, while Saruman becomes a traitor within the wizard order. The Witch-king is Sauron’s most powerful servant, not Sauron himself.