As Isildur rode away from the battlefield, he remembered with bitterness the look on Elrond's face when he had refused to destroy the One Ring. It was his weregild, his just recompense for the death of his father.
And yet the Elf had looked so pale, so scared, as he insisted that it should be destroyed, and heard the answer "No". Could it be...? No. The One Ring was his by right, and would remain so beyond the ending of his life, passing to his firstborn son, and his son after him.
Ignoring the doubts clamouring in some small part of his mind, Isildur rode on, keeping his eye on the forest around him.
Regardless of his wariness and caution, the attack took him by surprise. Before he knew it, there were Orcs all around, and a good half of his personal guard lay dead. Desperate to escape, and more importantly, to keep the Ring out of their hands, Isildur spurred his horse away from the fight, towards the river.
It was most likely a matter of pure chance that his horse's hoof slipped on a wet stone, and Isildur fell from the saddle. As he crashed into the water, he felt the first arrow thud into his back, and knew that his death had found him.
However, the sight of the One Ring, his precious heirloom, slipping from his finger and falling to the sand of the riverbed eclipsed even this knowledge. It was lost to him, and he would never get it back. And in that moment, in his last moments of life, Isildur son of Elendil knew despair.