In J.R.R. Tolkien’s legendarium, Sauron is the evil being who created the One Ring.
Sauron forged the One Ring in the fires of Mount Doom during the Second Age. He placed much of his power into it so that he could dominate the other Rings of Power and control their bearers. The ring’s destruction therefore required the destruction of a crucial part of Sauron’s strength.
Sauron served the first Dark Lord, Morgoth, before becoming the principal enemy of the later ages. This relationship sometimes causes confusion: Morgoth is the earlier and more powerful rebel, while Sauron becomes the central antagonist of The Lord of the Rings.
Sauron is also known as the Necromancer during the events leading into The Hobbit. In The Lord of the Rings, his physical form is largely absent, but his will and influence dominate the story.