Em ROTK, capítulo 2 do livro cinco, "A passagem da companhia cinzenta", Aragorn diz a Gimli e Legolas que ele olhou para a pedra de Orthanc e conversou com Sauron. Ele diz que Sauron "me viu, mas de outra forma que você me vê aqui". Qual foi esse "disfarce"? E por que Aragorn apareceria dessa maneira quando ele também se revelou a Sauron como herdeiro de Isildur?
Acho que a resposta para sua pergunta pode ser encontrada no contexto das palavras que você citou:
'You have looked in that accursed stone of wizardry!' exclaimed Gimli with fear and astonishment in his face. 'Did you say aught to - him? Even Gandalf feared that encounter.'
'You forget to whom you speak,' said Aragorn sternly, and his eyes glinted. 'Did I not openly proclaim my title before the doors of Edoras? What do you fear that I should say to him? Nay, Gimli,' he said in a softer voice, and the grimness left his face, and he looked like one who has laboured in sleepless pain for many nights. 'Nay, my friends, I am the lawful master of the Stone, and I had both the right and the strength to use it, or so I judged. The right cannot be doubted. The strength was enough - barely.'
He drew a deep breath. 'It was a bitter struggle, and the weariness is slow to pass. I spoke no word to him, and in the end I wrenched the Stone to my own will. That alone he will find hard to endure. And he beheld me. Yes, Master Gimli, he saw me, but in other guise than you see me here. If that will aid him, then I have done ill. But I do not think so. To know that I lived and walked the earth was a blow to his heart, I deem; for he knew it not till now. The eyes in Orthanc did not see through the armour of Théoden; but Sauron has not forgotten Isildur and the sword of Elendil. Now in the very hour of his great designs the heir of Isildur and the Sword are revealed; for l showed the blade re-forged to him. He is not so mighty yet that he is above fear; nay, doubt ever gnaws him.'
e muito mais tarde, Gandalf diz:
' As Aragorn has begun, so we must go on. We must push Sauron to his last throw. We must call out his hidden strength, so that he shall empty his land. We must march out to meet him at once. We must make ourselves the bait, though his jaws should close on us. He will take that bait, in hope and in greed, for he will think that in such rashness he sees the pride of the new Ringlord..."
(Minha ênfase.)
Ele deve ter se mostrado o herdeiro de Isldur. E Sauron tinha boas razões para temer o herdeiro de Isildur: Isildur arrancou sua mão quando Sauron ainda teve o anel. Este herdeiro poderia controlar o poder (por menor que fosse) de Gondor. E esse herdeiro era um homem de grande poder - ele foi mano-a-mano (por assim dizer) contra Sauron e venceu.
Então Sauron sabia que o Anel estava no exterior, que o Herdeiro de Isildur vive, não era um fraco, mas poderia desafiá-lo a um duelo de vontades e ganhar - e em breve poderá ter o anel. Aparecendo como o herdeiro de Isildur, Aragon mostrou a Sauron seu pior pesadelo.
A excelente resposta de Marcos ressalta que Aragorn se apresentou a Sauron no disfarce do herdeiro de Isuldur, e não apenas em um guarda do norte.
No entanto, a palavra "disfarce" refere-se à aparência física e à identidade. A aparência de Aragorn quando ele fala com Gimli e Legolas é alterada; ele parecia mais velho, sombrio e cansado.
The king mounted his horse, Snowmane, and Merry sat beside him on his pony: Stybba was his name. Presently Éomer came out from the gate, and with him was Aragorn, and Halbarad bearing the great staff close-furled in black, and two tall men, neither young nor old. So much alike were they, the sons of Elrond, that few could tell them apart: dark-haired, grey-eyed, and their faces elven-fair, clad alike in bright mail beneath cloaks of silver-grey. Behind them walked Legolas and Gimli. But Merry had eyes only for Aragorn, so startling was the change that he saw in him, as if in one night many years had fallen on his head. Grim was his face, grey-hued and weary.
The Lord of the Rings Book Five, Chapter 2: The Passing of the Grey Company
Page 778 (Houghton Mifflin Harcourt; Single Volume 50th Anniversary Edition)
Aragorn teria mostrado a Sauron uma aparência muito mais imponente. Não apenas ele não teria mostrado o cansaço que Merry viu (que foi o resultado de sua luta com Sauron), como teria mostrado a Sauron os símbolos que provavam sua identidade.
Os tokens que ele tinha disponíveis incluíam as heranças de sua casa
E depois há aquela "grande equipe de cabelos pretos" que Halbarad carrega. Tolkein menciona isso várias vezes antes de finalmente nos dizer o que é quando Aragorn chega de navio ao Harlond.
and behold! upon the foremost ship a great standard broke, and the wind displayed it as she turned towards the Harlond. There flowered a White Tree, and that was for Gondor; but Seven Stars were about it, and a high crown above it, the signs of Elendil that no lord had borne for years beyond count. And the stars flamed in the sunlight, for they were wrought of gems by Arwen daughter of Elrond; and the crown was bright in the morning, for it was wrought of mithril and gold.
The Lord of the Rings Book Five, Chapter 6: The Battle of the Pelennor Fields
Page 847 (Houghton Mifflin Harcourt; Single Volume 50th Anniversary Edition)
Acho que teria sido ótimo se Peter Jackson tivesse incluído no filme uma foto de Aragorn mostrando a Sauron com a Estrela de Elendil na testa, Andúril na mão e o Royal Standard exibido atrás dele. Seria um contraste interessante com a aparência cansada de Aragorn depois.