A fictional character from J. R. R. Tolkien's universe of Middle-earth, Arwen Undomiel (Arwen Evenstar) is the betrothed of Aragorn in Tolkien's The Lord of the Rings. She is the daughter of Elrond Peredhil and Celebrían (and therefore grand-daughter of Galadriel). She rejects her Elven immortality in order to marry Aragorn and become his Queen.
The romance between Aragorn and Arwen is reminiscent of the romance between Beren and Lúthien, but as with many other tales of the Third Age, theirs is a pale copy of the deeds of earlier ages. Still, only one other marriage between human and elf is recorded in the annals of Tolkien. Arwen is mostly a minor character in Tolkien's Lord of the Rings, while she is nevertheless described in detail in the appendices after the third book, The Return of the King. She does however play a role in the plot which is disproportionate to the number of scenes in which she appears. When Éowyn falls in love with Aragorn it is his fidelity to Arwen that forbids him from reciprocating, thereby motivating Éowyn's subsequent actions during the Battle of the Pelennor Fields which have major repercussions for the defence of Middle-earth.
Arwen plays a major role in the Peter Jackson movie version of the three books, since cinematically it is necessary to establish her role in the plot more firmly. In the 2002 film version, but not the books, she sneaks up on Aragorn, treats Frodo with athelas, and takes him by herself on horseback where she thwarts the Black Riders at Bruinen with a sudden flood. (In the book, it was Glorfindel who put Frodo on horseback and sent him alone to flee the black riders, and Elrond and Gandalf who arranged the flood.) During this flight and elsewhere in the movies Arwen weilds the sword Hadhafang, which was once wielded by her father.
Some argue that the substitution of Arwen for Glorfindel is a major departure in the film version of the story, and less acceptable even than that of the substitution of Legolas for Glorfindel in the Ralph Bakshi version. As with other changes in character and plot, this is a common and unavoidable hazard encountered in the adaptation of a story from a literary to a film format.
In the movie trilogy, Arwen is played by Liv Tyler.
Note: Arwen means "noble woman" in Sindarin