Théoden

In the fiction of J. R. R. Tolkien, Théoden was king of Rohan in Middle-earth at the end of the Third Age, and played a key role in the events of The Lord of the Rings.

Warning: Wikipedia contains spoilers

Misled by Gríma (or Wormtongue as most others in the Mark called him), Théoden initially rebuffed Gandalf's advice.

The uncle of Éowyn and Éomer, Théoden was killed in the Battle of the Pelennor Fields by the Witch-King of the Nine Ringwraiths, and avenged by his niece Éowyn.

Peter Jackson's film The Lord of the Rings: The Two Towers (2002) deviates from Tolkien's story by having Théoden actually possessed by Saruman rather than simply deceived by Gríma.