Tolkien writes she fell into depression since she longed to win renown in battle - more so because she was noble - but being female, her duties were reckoned to be at Edoras. When Théoden's mind was poisoned by his adviser Gríma Wormtongue, Éowyn was obliged to care for her uncle, and his deterioration pained her deeply. To make matters worse, she was stalked by Gríma.
However, when Gandalf arrived he healed Théoden from Wormtongue's corruption, and Éowyn became infatuated with Aragorn. It soon became clear that Aragorn could not return her love (though he did not mention his betrothal to Arwen), and would not allow her to join him in going to war. As Aragorn pointed out, her duty was with her people; she had to shoulder the responsibility of ruling Rohan in Théoden's stead of when the war-host of Rohan went to war. Aragorn also said her duties were no less valiant. Likening her situation to a "cage", Éowyn said she feared
"...[t]o stay behind bars, until use and old age accept them, and all chance of great deeds is gone beyond recall or desire."
Frustrated by unrequited love for Aragorn and longing for death in battle, she disguised herself as a man and under the alias of Dernhelm, travelled with the Riders of Rohan to the Battle of the Pelennor Fields outside the White City of Minas Tirith in Gondor, carrying with her Merry, who had also been ordered to remain behind.
During the battle of the Pelennor Fields, she confronted the Witch-king of Angmar, Lord of the Nazgûl, after Théoden was injured. The Witch-king boasted that "no living man may hinder me," referring to the 1,000-year-old prophecy by the Elf-lord Glorfindel, foretelling that the Witch-king would not "fall by the hand of man". Éowyn then removed her helmet and declared:
The Witch-king was killed after Merry stabbed him behind the knee and Éowyn stabbed him "between crown and mantle".
Éowyn was severely injured in this fight (her shield-arm was broken) and believed dead until Prince Imrahil of Dol Amroth realized she still lived. Because of the poisonous effect of the Black Breath of the Nazgûl and her hopeless love for Aragorn, she faced near-certain death and was brought up to the Houses of Healing together with Merry. However, she was treated in time by Aragorn. Éomer, while not blaming Aragorn, believed that unrequited love was at the root of her depression. Aragorn answered that she loved Éomer more truly than him, as her feeling for Aragorn was largely fantasy about the idea of Aragorn as a great leader and warrior representing the heroic life she could not have; and Gandalf pointed out the deeper roots of her depression.
While recuperating in the Houses of Healing, she met Faramir, with whom she soon fell in love, understanding that her previous "love" for Aragorn was more of hero-worship. Her outlook on life also changes:
"Then the heart of Éowyn changed, or else she understood it...
...'I will be a shieldmaiden no longer, nor vie with the great Riders, nor take joy only in the songs of slaying. I will be a healer, and love all things that grow and are not barren.' "
After the demise of Sauron, the happily wedded couple settled in Ithilien, of which Faramir was made the ruling Prince by King Elessar (the name with which Aragorn ascended the throne of the Reunited Kingdom). Faramir and Éowyn had at least one son (likely Elboron), and their grandson was Barahir, who wrote The Tale of Aragorn and Arwen in the Fourth Age. Tolkien nowhere gives the cause and date of Éowyn's death.
Éowyn is described to be very beautiful; she was tall, slim, pale, and graceful, with long golden hair. In temperament she was idealistic, spirited, brave, high-minded, and lonely.
Éowyn means "horse lover" or "friend of horses" in the Old English language (the language Tolkien used to represent Rohirric) - Eoh- meaning "horse" (which Tolkien renders Éo-) and -wyn meaning "joy".
The first syllable of Éowyn sounds like "eh-ah," with the "ah" just barely pronounced. As in Scandinavian or Finnish, the y in the second syllable is the same sound as the German letter ü or the French u. The actors in the Lord of the Rings film trilogy consistently pronounce her name as well as the names of Éomer and Théoden in a manner inconsistent with most reconstructions of Old English pronunciation.
Tolkien maintained Éowyn was not the character's actual name. Her real name in Rohirric is not given, but it, as well as Éomer and Éomund, would have started with the element Lô- or Loh-, meaning "horse", the equivalent of Old English Éo-.
Although she never carried the title of princess, she was a niece to one King of Rohan and sister to another, as well as the wife of a Gondorian prince.
Éowyn's titles included the (White) Lady of Rohan, Lady of Ithilien and Lady of Emyn Arnen. She was also known as the Lady of the Shield-arm in recognition of her triumph over the Witch-king.
At one point Tolkien described Éowyn as "a stern Amazon woman". Later he wrote: "Though not a 'dry nurse' in temper, she was also not really a soldier or 'Amazon', but like many brave women was capable of great military gallantry at a crisis." (Here he alludes to Éowyn's statement to Aragorn: "But am I not of the House of Eorl, a shieldmaiden and not a dry-nurse?")
Éowyn also appears briefly in Ralph Bakshi's 1978 adaptation but does not have any dialogue.
In Peter Jackson's films The Two Towers (2002) and The Return of the King (2003), Éowyn is played by Miranda Otto. (The role was first offered to Alison Doody, who turned it down because of her pregnancy.) She was apparently promised to Gríma by Saruman in payment for being the wizard's spy in the royal court (as in the book). She sang the death-song for Théodred. In the extended edition of The Two Towers, Éowyn is shown discovering, to her astonishment, that Aragorn is a long-lived Dúnadan. In the original theatrical release of The Lord of the Rings: The Return of the King, Éowyn's injuries after fighting the Witch-king were less severe than in the novel: she is conscious but hurt, as opposed to unconscious. In scenes added in the Extended Edition of the film, she was near death: her brother found her and screamed in anguish because he feared that she was dead and later we see her being healed by Aragorn.
While she did disguise herself in the film to ride into battle, she never took on the name "Dernhelm," and the audience is always aware of her true identity. In the extended version, Théoden noticed her carving through the enemy but it is not clear if he realised that it is his niece. The production team stated that while in a book it was easy to disguise Éowyn's identity, in the medium of cinema the audience could visually tell that it was she, and it would have strained the credibility of the scenes to try to make it a secret and would have made Merry look foolish.