This is written as a companion piece to Isildur's Heir and provides a bit of back story for Eradan's betrayal. That being said it does contain major spoilers so probably read that story first. :) Cheers!
Darkness took them,horse and horseman;
Hoof-beats afar sank into silence; so the songs tell us.
Eradan of the Dunedain wept for the passing of his lord, though passing seemed to gentle a word for Arathorn's end. Slaughtered by orcs; those were the tidings borne through all the lonely villages of the Rangers and to every bright hearth. The chieftain was dead and they were alone with none to lead them. A shadow, long held back by the line of Elendil, seemed now to break across the land and its people.
Eradan watched, uncaring, as his people faltered and fell; all the grief in his heart was spent to mourn the loss of one who had been as a brother to him. Their paths had grown apart of late, but the love and loyalty Eradan held for Arathorn had not wavered and so he spent his days in grief and had little thought to give he suffering of the Dunedain. When grief at last was spent he fell instead to anger and it burned within his blood. Hatred filled him for the fell creatures who brutally slew his lord and friend, but slowly his anger turned in part to those who failed to save his chieftain.
The sons of Elrond became hateful in his mind and in his sight, for it had been they who rode out with Arathorn in the hour of his death. They had not saved him even though their prowess as warriors was lauded by both elves and men. Moreover they had wronged Arathorn by taking his son from his kinsmen. The last of Elendil's line would now live far from his people and the heritage of his line and it was this that at last hardened the anger in Eradan's heart to bitterness. The hope of men was stolen by elves to claim it as their own and who could say if Aragorn would ever return to his true people.
The years passed in slow bitterness for the ranger as he watched for the return of his lord's son; ever hoping that the boy would be restored to his people. But, as the years became decades, his hope seemed without foundation and despair closed cold hands around him. Winter passed into Summer and with each season his hatred of the elves grew and festered within him, tinting his thoughts with madness. If ever the sons of Elrond dared to face him he resolved to slay them for their crimes against Arathorn and his son. He gave up hope and fell into darkness of spirit which seemed unbreakable.
Arathorn's son returned in the spring. Riding into the village on a great steed he seemed like a king of old returning to his kingdom and for a moment Eradan allowed himself to hope. The son of the North had returned, but with him rode the sons of Elrond and hope grew bitter within him. The young man who returned to them was not Arathorn's son; he was Elrond's. The elves who had failed his father had failed him in his raising and he seemed more of their race than of his own.
Eradan wept at the loss as he had wept at Arathorn's death; the last heir of kings was no more. The aging ranger turned his steps away from his home and his people for a time and wandered through the wilds, unable to face his chieftain. He could hold no love nor loyalty to one so caught up in the affairs of elves. And so it was that Eradan's path crossed with that of the wizard Saruman.
He sought wisdom and peace in the wizard's company, but found only a malice to match his own. When their paths once more diverged he heard Saruman's whispered words of treachery still in his mind and his last tenuous hold on sanity was abandoned. The mind of the wizard was strong and his own was weak; Saruman fanned his hatred for the elves into a mighty flame and turned his wrath upon Aragorn. Eradan returned to his kin in the guise of a loyal adviser; he swore oaths of loyalty to the chieftain whose very life he abhorred. And all the while he sowed seeds of discord where ever he tread and to him he gathered all those who harboured discontent. All the while Saruman whispered within his mind and strengthened his madness with lies. All that was good within him burned in the fires of his and the wizard's hatred and one thought grew above all others. He must kill Isildur's heir; he must kill Aragorn to save him from the corruption of the elves.
Saruman awaited him when at last he rode to Isengard and with him he bore the news of Aragorn's death by his own hands. The wizard smiled but was silent and silent too were the whispers of madness within the ranger's mind. He shuddered at the memory of his treachery and wept with the sorrow of his lord's death. He wandered in the wilderness and cursed the weakness of his spirit and at last he saw the truth, but too late. The elves were not to blame, nor was Aragorn, it was by his madness and the cunning evil of Saruman that the hope of men was slain. In that knowledge he wandered alone and knew not even death had the power to release him from the torment of his folly.
Through many years Saruman held the knowledge of Aragorn's death as his greatest triumph. He had done through cunning what others had tried and failed to do; by whispered madness in the mind of a ranger he had slain Isildur's heir. But even in his hour of triumph, on the very eve of his final victory all he sought to achieve was torn from him. He heard ill tidings of a ranger, journeying with Gandalf to the defense of Rohan and his heart grew heavy within him. The line of kings was broken by his actions and the folly of a ranger, and yet from the ashes a flame had sprung. The line was broken, and yet, Aragorn still lived. Saruman saw that his doom had come upon him and none now held the power to save him.