Beautiful, poignant story.

I enjoyed your take on this fateful moment, especially the opening vision of Elrond running sand through his finger and then that image returns again when Elros explains about the cube of sand and the grains like notes in the song of Arda and his desire to multiply the notes in his descendants.  Lovely.

"Will you teach my children what you learn?" Elros said.

That got me. Everything leading up to it was so beautiful and heartwrenching and then we get Elrond's determination to help his brothers descendents through ages and ages. Good story. 

Wow. This is so poignant. It really is a horrible choice, and I love how conflicted Elrond is over it. He's so young (well, relatively speaking...) and he seems so weary already.

"He was sure he did not want to be separated from his brother, sure he did not want to be bound to Arda forever, and equally sure that the wisdom he craved would take far more than a mortal lifetime to master, and longer than that to transmit."

It almost sounds like Elrond loses either way. But I love that learning, and teaching, is so important to him. It makes sense for him. The background, about how he took it upon himself to make sure Elros knew their history, was a great addition to that.

The conversation between Elrond and Elros shows how well they know each other, which just makes their parting so much more tragic.

There were so many gorgeous lines in this story, but I think this is my favorite, because it's so hopeful and a little haunting:

"Elros Tar-Minyatur led the first of the ships of men to the realm of Numenor, where they established a kingdom to last an age and a people to last beyond it."

Great story!