When Gandalf, Aragorn, Legolas and Gimli go to Rohan, Saruman is controlling Théoden’s mind. In Peter Jackson’s film adaptation of The Two Towers, Saruman’s influence has weakened and prematurely aged the king of Rohan. Gandalf breaks the spell, restoring Théoden’s strength and judgment.
The story also involves Gríma Wormtongue, Théoden’s chief adviser. Gríma secretly serves Saruman and manipulates the king through lies, persuasion and possibly poison. This creates a common source of confusion: Saruman is the supernatural force behind the control, while Gríma is the human agent carrying out his plans.
In Tolkien’s book, the situation is presented more as Gríma’s poisonous counsel and Saruman’s influence than as a visibly enacted possession. The film makes the control literal and dramatic, helping viewers understand why Théoden appears helpless until Gandalf intervenes.