I have tried really hard to keep this canon, but I'm not at all an expert, so if you see any mistakes in my history or timeline, or if you think something's out of character, please don't hesitate to pull me up about it. Hope you enjoy.
The palantír was cool and smooth against his hands, murky and still. Denethor slowly turned his head to stare into its ageless depths, clearing and readying his mind for the ordeal that he knew was to come. His stomach tightened with nervous anticipation, and he forced his breath to slow, before slipping into the depth of the palantír...
At first all he saw was a blue grey haze, shifting and swirling like smoke on a breeze, but slowly the fog began to clear. He saw Minas Tirith rising from the plains, framed by the mountains and the deep blue sky. This was where it always began; no matter how hard he tried to start somewhere else, the first thing he saw was always his city as it had looked in the morning so many years ago, as he rode back into his home... He brushed the picture from his mind, and focused on Imladris. For a moment he glimpsed a village somewhere in Gondor's northern borders, then suddenly the valley swirled into view, hazy at first, but becoming clearer as he focused in closer, unconsciously moving a little further from the glowing ball as his vision cleared and the trees moved closer into veiw.
Rivendell itself was shrouded in mist, as it always was. At first he had thought it was just caused by the river, but it never cleared, and after probing it from every angle he had come to the conclusion that it was deliberately devised by Elrond to keep out prying eyes. It was wise, yes, but also maddening. The last time he had seen his son was almost a month before as he entered Imladris, and Denethor had no idea whether he was still there or had already turned homeward. Searching here was futile, yet he longed to see his son again, to know if he was safe...
After vainly scanning the forest for a few moments, he sighed and turned his gaze southward, focusing into Hadar, the capital of the Harad. The city was busy, and throughout the streets soldiers wandered, their purses jangling at their belts and attracting the instant attention of the street vendors. Denethor's gaze barely swept over them; he had seen them there before and knew there were now over four thousand men gathered in the capital, crowding it to a point where the city could not sustain them for many more weeks.
The Haradrim were about to march to war and Denethor knew where they would come. Gondor and Harad had long been enemies; he had no hopes of them allying themselves with Gondor now with such a force as Mordor arrayed against them. The armies of Gondor could withstand easily the forces of Harad and crush them were they to attack alone, it was only this that had brought them the long ages of peace. Yet now the Haradrim were coupled with the orcs of Mordor, the armies would be phenomenal, and he feared that Gondor could not withstand such an attack.
Suddenly Hadar disappeared. A vision of himself appeared standing and shrouded in dark smoke. He struggled to control the glowing orb, trying to shift his gaze, but he could not. At last he forced himself to relax in order to save his strength, and slowly the smoke cleared. He realised he was on the wall at the top level of in the White City as it burned around him. Far below the plains were dark with men and orcs; with the heightened sight of the palantir he could see the forces of Mordor pour into the lower levels of Minas Tirith, burning and killing as they went.
With an inhuman effort he pulled his gaze away, and turned toward the citadel. There he froze, his gaze riveted to the White Tree, which stood, wreathed in flames. Its guards were still standing still as statues around it, facing outward, not seeming to notice the ruin to the treasure of Gondor they were supposed to be guarding.
With a cry of anguish he again wrenched his gaze away, but the palantír seemed to possess a mind of its own, and his gaze was immediately pulled toward the East, while a deep, rumbling whisper grew and coursed through his head. He felt the presence of Sauron, battering his mind, trying to take control of the palantír and drawing Denethor deep into the dark land... Shadows misted around him, invading his mind and clouding his thought with despair, and he heard the screech of a Nazgul ripping through his senses and almost overpowering him. He felt the Dark Lord laugh in triumph, but with a last effort he pulled himself away, wrenching his hands from the palantír with such force he threw his body halfway across the room. He dropped to his knees and knelt for a long time on the cold marble floor until he finally stopped trembling. He felt so old, wasted, drained. Every time he used the palantír Sauron seemed to get stronger, or he weaker. He had had no control over where it took him...
Slowly he pulled himself to his feet and walked out to the tower balcony. He stopped beside the door to pull a heavy cloak over his mail-clad shoulders before stepping out into the biting wind. Immediately he felt the load lift from his shoulders, and peace swept over him. He moved to the wrought iron railing, staring far down into the streets of Minas Tirith. After the dizzying experiences of the palantir, the high tower seemed very solid and safe. For a while he stood, clearing his mind and letting the wind blow through his hair and his soul, feeling peace wash through his veins.
The palantir had aged him a lot, both physically and mentally. His hair was now streaked with grey, and the lines on his face had deepened almost overnight. And inwardly, he felt tired, heavy and old. Now he wished only to die, to go, to be at peace with his wife wherever men's souls were doomed to go. Yet Gondor needed him, the White City needed him, his people needed him, and he would not forsake them in their hour of need. For an hour of need it was, with such armies amassing against them. Gondor could fall at any moment, and his days were occupied with frantic preparations for war.
He glanced toward the east, half expecting to see right into Mordor, where orc armies were massed on the plains around Barad Dur, and to feel the crushing will of Sauron battering his. Yet the only trace of Mordor was the faint but constant shadow lurking over the mountains. The Pelennor fields were lush with crops, everything was green and fresh, the people busy. They knew nothing of the shadow that their steward fought throughout long hours in his White tower, or the armies that soon would be gathering against them. To them they were only rumours, things to be put aside before the greater concerns of bringing in the harvest. Responsibility for protecting the city fell on him.
With a sigh of exhaustion he turned back into the tower. The palantír stood uncovered in the middle of the large, circular room, and Denethor quickly draped a dark, silken cloth over it, cursing himself for his neglect. He did not want prying eyes in this room. Wearily he walked over to a low chair at the end of the room and dropped into it. An image of the first time he had been in this room flashed into his mind.
He was seventeen when his father had summoned him to the tower. Throughout Denethor's life his father had rarely called him into his presence for anything but a telling off. He was a wilful boy and usually fully deserved it, yet after examining his conscience he could not recall anything unusually sinful in his recent past, and besides, never had Ecthelion called him to the tower. So it was with a mixture of excitement, curiosity and trepidation he climbed the many steps to the tallest tower of the citadel that day.
The room had always been the private retreat of the stewards, and before them, the kings, and never before had Denethor entered it. He banged loudly on the door, and after a moment heard his father's voice bidding him enter. The door opened noiselessly at his touch, and he stood in the doorway for a moment, taking in his surroundings. The room had eight sides. Like an octagon, he thought, pleased he'd remembered something from his long and fearfully boring geometry lessons.
The walls were crafted of smooth, white marble, rising up and curving inward high above to meet in a point directly above a large table in the centre of the room. In the centre of each wall was a pair of large, black double doors under huge grey arches carved with detailed patterns and runes. Light streamed down from small windows set high in the walls, falling on the couches and tables scattered around the walls. In addition, on either side of each doorway was an iron torch in a beautifully carved bracket, which cast flickering shadows up the walls.
Directly opposite him the doors were thrown open revealing a wide balcony where his father stood, leaning lightly on the railing. As Denethor watched him, he turned and beckoned impatiently to his son. Denethor hurried across the room and under the carved archway to stand beside his father. The view was amazing. The city lay miles and miles down, shining white in the sunlight, surrounded by the golden, green and brown of the fields around the city. To his left he could see the ruins of Osgiliath on the banks of the river, and in every direction massive mountains rose from the rolling plains. He drew in his breath in a gasp of amazement at the sight. Everything was set out before him like a beautiful map, drawn by the finest of elven artists...
"Do not be afraid," Ecthelion snapped irritably, wrongly interpreting his gasp to be one of fear, "You will not fall."
"I am not afraid, father! I am not a child." Denethor burst out indignantly, then immediately regretted it as his father shot him a glance of anger and turned away.
"Forgive me, for I meant no disrespect!" he cried, following his father back into the tower. "Please, father, won't you at least tell me why you called me here?"
Ecthelion sat down in one of the low chairs, and regarded his son stonily. "I called you because I thought you ready to learn what it is to be a steward of Gondor. I see that I was wrong, as you cannot even listen submissively when I speak to you."
"Forgive me, father, please."
The steward rose from his seat and placed his hands on his son's shoulders, gazing into his eager eyes. He absently noted that Denethor had grown a lot, the boy was now only a little shorter than his tall father. Finally he nodded. "Come then," he said, moving out onto the balcony, this time from the doors to his left. "The Haradrim are attacking," he said calmly. "They have just appeared on the western hills. We have had no warning; the people are all still in the fields."
"What!" Denethor asked incredulously.
"What? Is that all you can say? If they were really attacking, asking your captains to repeat themselves is a waste of valuable time," the steward told him sternly, but with a glint of humor in his grey eyes. "Think boy, use your imagination, for you are in charge of defending the city. You have five hundred horsemen and about one thousand footmen that can be gathered on short notice. What are you going to do? These are the kind of scenarios you will face as steward, you have to be ready for them at any moment. I am waiting your command."
Ecthelion drilled his son for a couple of hours, and while at first Denethor was hesitant and questioning he gradually grew more confidant and began almost to enjoy the strategic battle of wits against his father's imaginary armies.
At last the steward halted the battle and dismissed his son to have his dinner. As Denethor entered the tower, something shrouded in cloth on a pedestal caught his attention. He stopped before it, curious as to what should have to be so hidden even in his father's private room, but hesitant to touch it and perhaps arouse his father's anger.
Ecthelion followed his gaze and answered his unspoken question. "It is a palantír, a Seeing Stone," he said solemnly. "A treasure of Gondor from the ancient realm that is said to have been placed here by Elendil himself. Have you heard of this thing?"
"Yes, I have read of it," Denethor said, his eyes shining as he gazed at his father. "I have heard that those who look into it can see anything they wish. I knew there was once one here, but I didn't realise you had it – well, that it was still here. Have you looked in it, father?"
"No, I have not," Ecthelion said sharply. "It is a tool, not a plaything. Many of the palantíri are lost, we do not know who might hold one. Tell me, boy, why that means we should be reluctant to use them."
Denethor frowned. "Because if an enemy had one, they might see you using it and realise you could see what they were doing?"
"No. Well, they might, but it wouldn't make much difference. Think boy, to use a palantír you must open your mind, and the things you see are in your head and controlled by your thoughts."
Denethor stared at his father, light dawning in his head as he processed this.
"If an enemy more powerful than you held a Seeing Stone," Ecthelion continued, "he could see into your mind, even control your thoughts. Imagine if this happened to an unwise steward. Through him the enemy could control the city, and Gondor would fall, and from thence the whole of Middle Earth. As such, the palantíri are very dangerous, and should not be used again until all are accounted for. Do you understand?"
Denethor wordlessly nodded and followed his father down the long, spiral staircase. After a moment Ecthellion started to talk again, his voice echoing eerily against the walls. "We know for certain there was a palantír in Minas Ithil before it fell; it was used several times for contact between our cities in emergencies. When Minas Ithil was taken by the Nazgûl, several of the last defenders escaped with it and fled at the last. They were found later dead, and we know not whether they managed to hide the palantír before they were overtaken, or if it was captured, or overlooked and picked up by some scavenger not knowing what it was. Stranger things have happened. Yet the palantír may even be in the hands of Sauron himself, and since that time our stone not been used."