Denethor was the twenty-sixth Steward of Gondor during the Third Age of Middle-Earth. He had an ancestor, also named Denethor, who was the tenth steward of Gondor and the first to confront the Uruk-Hai. Denethor was once a wise, valiant, proud and noble man, but paranoid at the same time. He never trusted the wizard Gandalf, fearing that he was plotting to supplant him with the heir of Isildur. Denethor was married to Finduilas of Dol Amroth and she bore him two sons, Boromir and Faramir. But due to complications, she died in child-birth when Faramir was born and afterwards Denethor became grim, withdrawn and grew to resent Faramir.
During Denethor's years, Sauron's forces were stirring in Mordor, once again growing strong. In order to learn of Sauron's plans, Denethor began to look into the palantir of Minas Tirith, and while he did gain the knowledge he was seeking, he became ensnared by Sauron, who was able to partially control what Denethor saw.
After the death of Boromir, and with the armies of Mordor prepared to invade, Denethor fell into grief and despair and sent Faramir on a suicide mission to re-take Osgiliath from Mordor. Faramir survived and was brought back half-dead by Prince Imrahil, Denethor fell into madness, and upon gazing into the palantir one final time, he saw the capture of Frodo Baggins at Cirith Ungol (and believed Sauron had found the One Ring) and had a vision of a fleet of Corsair ships heading towards Minas Tirith. At that moment, Denethor completely lost his mind. He ordered that a great pyre be built, he and Faramir would burn.
Denethor was stopped from burning Faramir by Gandalf and Beregond, but they failed to save him, and grasping his palantir in his hands, Denethor burned to death.