Sauron is the Maia who becomes the Dark Lord in Tolkien’s legendarium. Before his corruption, he was called Mairon and served the Vala Aulë, associated with craftsmanship and making.
Sauron later joined Morgoth, the first Dark Lord, and became one of his most powerful servants. After Morgoth’s defeat at the end of the First Age, Sauron eventually rose as the principal enemy of the Second and Third Ages.
He created the One Ring to control the other Rings of Power and sought dominion over Middle-earth. His physical forms were destroyed or changed at different points, but his will and power continued to threaten the world.
Sauron is sometimes confused with Saruman because both are powerful supernatural beings and both become corrupted. They are not the same character: Saruman is a wizard, while Sauron is a Maia who predates the Wizards’ arrival in Middle-earth. Gandalf is also a Maia, but he opposes Sauron.