In Rohan, Aragorn was joined by some of his kinsmen, Rangers from the north. With them were the two sons of Elrond Half-Elven:
‘I have thirty with me,’ said Halbrad. ‘That is all of our kindred that could be gathered in haste; but the brethren Elladan and Elrohir have ridden with us, desiring to go to the war.’
Two nights later, Aragorn led this company, plus Legolas and Gimli, through the door to the Paths of the Dead, a dread place:
The company halted, and there was not a heart among them that did not quail, unless it were the heart of Legolas of the Elves, for whom the ghosts of Men have no terror.
The sons of Elrond haven’t gone missing. Elladan at least is still there:
Aragorn had brought torches from Dunharrow, and now he went ahead bearing one aloft; and Elladan with another went at the rear.
Either there is something deeply subtle about the nature of Elladan and Elrohir, or Tolkien made a mistake. Which is it?
All quotes from Book Five, Chapter II, “The Passing of the Grey Company”.