Gríma

Gríma and later called Gríma Wormtongue was the advisor of King Théoden, before being exposed as an agent of the wizard Saruman.

Servant of Saruman
He is first seen in Isengard when Gandalf is imprisoned by Saruman in the Tower of Orthanc, when he hands the evil wizard his staff.

Treachery Revealed
Gríma poisons the mind of King Théoden with ill words, telling him how Gandalf the Grey was an enemy and even tries to kill the wizard with a dagger. He is stopped and his lord freed from his evil spell. It is revealed that he had been in the service of Saruman for some time, spying on Rohan for his master. He flees back to Isengard.

Isengard is unleashed
Back in Isengard, Gríma and Saruman had amassed legions of Orcs from Moria to fight for them against the Free Peoples of Middle-earth. The wizard unleashes his army upon the Men at the Battle of Helm's Deep.

Trivia

 * In Ralph Bakshi's The Lord of the Rings, he is portrayed much smaller than his book counterpart.
 * His motivation behind poisoning Théoden's mind in the books is that Saruman may have promised him Éowyn, the king's niece, as wife. In Bakshi's adaptation however, Wormtongue seems to have always been in league with the forces of Darkness, as he is briefly seen earlier handing Saruman his staff, before Gandalf is imprisoned.