Written for Back to Middle-earth Month 2012, prompts B9: "Cause of Death: Torture" and "The Steward and His Sons: Son and Heir."
When Boromir was eight years old, he asked his father, "Why do you not sit on the high seat in the great hall? It is much finer than your chair."
"The throne is reserved for the King," Denethor said, frowning.
"But you rule the City, my father," Boromir said eagerly. "You should be the King."
"I am not King, but Steward," Denethor answered. "But to be Steward is a place of much honor. Do you understand why it is that I, being the Lord of the City, am called Steward and not King?"
Boromir shook his head. Denethor looked upon him sternly. "You are my son and heir," he said, "and you are of an age where you should know these things. Come." He rose to his feet. At his father's sign, Boromir obediently followed.
"To be a steward," Denethor said as they walked, "means to hold something in trust for another." Seeing that Boromir did not fully understand, he added: "To take care of it, and to keep it without harm. Since I am Steward, the City of Minas Tirith – indeed, all Gondor – is my charge, and someday it will be yours." Boromir's eyes shone with pride at his father's words, and he followed eagerly.
Denethor led him to a library and pulled out a thick volume. It gave off a puff of dust as he set it on the table. "Gondor once was ruled by Kings," he said. "They were descended from Elendil and his son Anárion."
"Elendil fought the Dark Lord," Boromir remembered.
"Indeed he did, my son. And that is the same Enemy we still fight against today."
Denethor opened the book and traced his finger down the page. "In time, the line of the Kings failed. The last King of Gondor was Eärnur. In his rashness, he rode forth with a small following to meet the challenge of the Lord of Minas Morgul. He is foolish who trusts in the good faith of an enemy. Of their fate no certain word came back, but it is said that his men were slain and Eärnur died in torment. When the King did not return, his Steward—our forefather—ruled the City in his stead, and his son after him."
Boromir considered this, frowning. "But if the King is gone, why cannot you become King, my father?" It was not the last time he asked that question.