'I know only the little that Gandalf has told me,' said Frodo slowly. 'Gil-galad was the last of the great Elf-kings of Middle-earth. Gil-galad is Starlight in their tongue. With Elendil, the Elf-friend, he went to the land of—'
'No!' said Strider interrupting, 'I do not think that tale should be told now with the servants of the Enemy at hand. If we win through to the house of Elrond, you may hear it there, told in full.'
'Then tell us some other tale of the old days,' begged Sam; 'a tale about the Elves before the fading time. I would dearly like to hear more about Elves; the dark seems to press round so close.'
'I will tell you the tale of Tinúviel,' said Strider
Frodo begins to offer the little he knows about Gil-galad, but Aragorn stops him before he can say the name 'Mordor'. I am reminded of Gandalf inspecting the Ring, and saying that he won't say words in the language of Mordor in the Shire. Even the language of the place is, apparently, dangerous, and here, maybe even the name.
Sam, of course, always wants more tales of Elves, but I think Aragorn might be doing more than just telling a tale to pass the time when he chooses this particular story: the story of the woman who rescued her love from the dungeons of the Master of the Ring-wraiths. Aragorn does say the word 'Sauron' here, and he says it specifically in the context of Sauron's defeat:
Tinúviel rescued Beren from the dungeons of Sauron, and together they passed through great dangers, and cast down even the Great Enemy from his throne, and took from his iron crown one of the three Silmarils, brightest of all jewels, to be the bride-price of Lúthien to Thingol her father.
And that reminds me of the part of the story of Lúthien that Aragorn doesn't mention here, of Finrod's song battle against Sauron from the Silmarillion:
Then sudden Felagund there swaying
Sang in answer a song of staying,
Resisting, battling against power,
Of secrets kept, strength like a tower,
And trust unbroken, freedom, escape;
Of changing and of shifting shape
Of snares eluded, broken traps,
The prison opening, the chain that snaps.
Finrod, long ago, fought Sauron with song, (and lost) but I wonder if Aragorn, telling the tale of Lúthien here in the wilds with the Ring-wraiths lurking just beyond the firelight, is doing something similar. Is he telling a tale of one of Sauron's old defeats, and holding back or weakening the wraiths in the darkness with his words? It's interesting that he focusses the story strongly on Lúthien's victory (not Finrod's failure). It is only once Aragorn finishes his tale that the wraiths attack.