Born during the Stewardship of his grandfather Turgon, Denethor was a Gondorian noble who would see some of the most turbulent times in his country's long history, culminating in the War of the Ring itself. While still a youth of twenty-one, he saw Sauron re-emerge as the Dark Lord of Mordor, and start building his power on Gondor's eastern borders. Just a few short years later, Mount Doom burst into fire once again. In Gondor's time of need, a new hero arose, and for a time StewardEcthelion II (who had recently succeeded his father Turgon) was served by the mysterious captain Thorongil (who was long afterwards discovered to be none other than Aragorn of the Northern Dúnedain).
Nearly twenty years passed, and beneath the looming threat of Mordor, Denethor wedded Finduilas, the daughter of Adrahil of Dol Amroth. Together they had two sons, Boromir and Faramir. Soon after, Denethor's father Ecthelion died, and Denethor succeeded to become Steward Denethor II.
Just four years after Denethor succeeded to the Stewardship, Finduilas died at a young age for one of her line. For the next thirty years, Denethor ruled as a grim and serious Steward, and it later became known that for at least some of this time, he had made use of a palantír to gain knowledge, through which the malice of Sauron had set him on the road to insanity. In the end, the death of his elder son Boromir, and the imagined loss of his younger son Faramir, sent him mad. He burned himself to death with his palantír in his hands.
Denethor II ruled as Steward for thirty-five years, and was succeeded by his younger son Faramir.
Notes
1
'Denethor' was the name of a hero of the Nandor in the Elder Days, slain in the First Battle of the Wars of Beleriand during the First Age. It was common practice for members of the House of Húrin to take the names of ancient heroes, so it seems more likely that Denethor II, and his ancestor Denethor I, were named after this ancient Denethor, rather than being lithe and lank themselves.