Saruman is the wizard who becomes corrupted by Sauron’s power and ambitions. Originally called Saruman the White, he is the leader of the Istari and head of the White Council, an order created to oppose the Dark Lord’s return.
Saruman’s fall begins through his study of Sauron and the Rings of Power. Rather than remaining a guardian of Middle-earth, he starts seeking the One Ring for himself and proposes an alliance with Sauron—or a plan to challenge Sauron once he has gained enough power. When Gandalf refuses to join him, Saruman imprisons him in Orthanc.
The corruption is not simply mind control: Saruman chooses domination because he believes his knowledge and authority justify it. From Isengard, he industrializes the valley, cuts down trees, and breeds an army of Orcs and Men. The Ents later destroy Isengard, and Gandalf strips Saruman of his staff and place in the order.