Once the Ranger-General of Silvermoon, Sylvanas Windrunner was murdered by the death knight, Arthas, and brought back as a creature of undeath. After regaining her free will, she rebelled against the Lich King and the Scourge and became the founder of the Forsaken faction of undead, who are allied with the Horde. She has styled herself the Dark Lady, Queen of the Forsaken, an expansion of her original title, "the Banshee Queen".
As leader of the Forsaken and the Horde on the Eastern Kingdoms, Sylvanas is a military genius. The middle sister of Alleria and Vereesa Windrunner, she was Ranger-General of the high elf kingdom of Quel'Thalas. She fought valiantly against Arthas when he invaded Quel'Thalas, but during the fall of Silvermoon, Arthas raised her as an undead minion as a form of petty revenge. When the Lich King's hold on his minions weakened, Sylvanas, among other undead, regained her free will, and she led her forces against the dreadlords who had taken hold over Lordaeron, ultimately affirming her sovereignty by killing off two of the nathrezim and bending the third, Varimathras, to her will. She renamed her force of self-aware undead the Forsaken and declared herself their queen.
Sylvanas was a member of the prominent Windrunner family of the high elves. Her siblings were Alleria, Vereesa, and at least two other brothers (one named Lirath - as seen in the novel Beyond the Dark Portal - and the other name is not revealed). Her family lived at Windrunner Spire in the tranquil forests of Quel'Thalas. Sylvanas joined the rangers and eventually became their leader, rising to the rank of Ranger-General of Silvermoon, military leader of all high elven forces.
Sylvanas, the Banshee Queen, and leader of the Forsaken traveled to Quel'Thalas for her own reasons, but soon encountered the traitor Dar'Khan, confronting him in the ruins of Silvermoon. She aided Kalecgos in attempting to stop the traitor from claiming the Sunwell's energies; though she was initially unsuccessful, she distracted Dar'Khan long enough for Anveena to regain her power and vaporize him. Sylvanas agreed to keep Anveena's identity a secret in the interests of preventing Arthas from gaining the Sunwell's power.
Though in life Sylvanas was a high elf, her original model in the World of Warcraft was that of a tall female night elf, but this is because that was the closest model available before The Burning Crusade. During the Wrath of the Lich King beta she was given a retextured blood elf model, and by patch 3.0.2 she was given a unique undead high elf model more reminiscent of how she appeared in The Frozen Throne. Interestingly, the outfit she wears in the Sunwell Trilogy is more similar to those of Sentinel archers than the high elf rangers in the same book.