"The exact origins of "Lament of the Highborne" have been lost to antiquity. Certainly the melody pre-dates the War of the Ancients, in which the song served as a lament for the fallen: a tribute to their steadfast bravery against overwhelming odds. Millennia later, the high elves of Quel'Thalas sang the lament, largely unaltered, in the Troll Wars. Here, too, the elves ultimately proved victorious, but they would not remain so. A plague of undeath swept through the human nation of Lordaeron in the Third War and converted many citizens into mindless undead minions of the Lich King. This army--the Scourge--invaded elven lands and decimated the populace, some of whom were raised into undeath and forced to join the Scourge.

As leader of the last elven defenders, Sylvanas Windrunner suffered a terrible fate: her spirit was ripped from her body and transformed into a banshee. When the Lich King's power faltered, however, she and other undead were freed of Scourge control. She has since reclaimed her body and become queen of these undead rebels, the Forsaken. Although her heart is full of bitterness, she cannot forget the people she once loved... the kingdom she gave her life to defend. "

Source - http://www.worldofwarcraft.com/downloads/movies.html#lament.