Lady Sylvanas Windrunner, The Banshee Queen

Backstory from World of Warcraft Game Manual

Sylvanas Windrunner - Before the coming of the plague, Sylvanas Windrunner was the brave Ranger-General of Silvermoon. During the Third War, Prince Arthas invaded the elven kingdom of Quel'Thalas. Sylvanas led her forces in a desperate struggle against the undead Scourge, but Arthas eventually defeated the high elves and transformed Sylvanas into one of his banshee slaves. Cursed to mindless undeath, Sylvanas only regained her will when the Lich King's powers waned. Eventually she outwitted the Scourge and broke away with her rebel faction, which she came to be known as the Forsaken. Now she rules over the Forsaken from the Undercity of Lordaeron. Her goal is to take vengeance upon Arthas one day and find lasting freedom for her cursed people.

World of Warcraft Game Manual (Copyright ©2004 by Blizzard Entertainment.)

Backstory from Wikipedia.org

Sylvanas was a dedicated and loyal Ranger General of the High Elven kingdom of Quel'Thalas but was tragically transformed into a Banshee by the Death Knight Arthas and is now known as the Dark Lady, Queen of the Forsaken.

Sylvanas led the High Elf Ranger Corps in defending the borderlands of Quel'Thalas from the invading Undead Scourge while simultaneously attempting to summon reinforcements from the elf capital city of Silvermoon during the Third War. Though Sylvanas mocked him from the outset, Arthas crushed her initial defense at the Outer Elfgate, driving her further into Quel'Thalas. After destroying a bridge on the only route deeper into elvish territory and taunting Arthas further, Sylvanas moved deeper into Elvish territory to obtain more reinforcements. However, Arthas was able to bypass this problem by using goblin transports to fly over the river, where he obtained the pieces of the Key of the Three Moons, which he used to open the Inner Elfgate after defeating the resistance that stood in the way. Sylvanas then retreated back further to her main base near the elvish capital of Silvermoon. Again, she taunted Arthas and continued to attack him while sending messangers directly to Silvermoon to get reinforcements. Arthas, who had become a Death Knight in service to the Lich King Ner'zhul, stopped the messengers and defeated her small group of resistance before moving on to crush Silvermoon and the High Elf civilization. When her base was leveled and all of her forces slain, Sylvanas stood alone against Arthas. Even when she knew she was beaten, she mocked Arthas by saying "Finish it; I deserve...a clean death." However, Arthas was already enraged by Sylvanas' persistant taunts and her stubborn refusal to surrender. He retorted "After all you've put me through woman, the last thing I'll give you is the peace of death." Then Sylvanas realized that her doom was sealed and proclaimed "No, you wouldn't dare!" Arthas killed Sylvanas on the spot and using his dark magics granted by the Lich King, prevented her soul from escaping and raised her from the dead as a Banshee. Now a mindless killer in the service of the Lich King, he used her to aid in the destruction of the civilization she once protected. For the remainder of Warcraft III, the Lich King used Sylvanas to help destroy Lordaeron and claim it for the Scourge. Around the time of Archimonde's defeat, the Lich King rewarded Sylvanas by pulling up her corpse from the earth and allowing her to inhabit her dead and rotting body.

In the aftermath of the Burning Legion's defeat at Mt. Hyjal, Arthas, Kel'Thuzad and Sylvanas began eliminating the last remnants of human resistance in the land of the former human kingdom of Lordaeron. Sylvanas initially remained one of the mindless Scourge, a slave to the Lich King's will; but regained her self awareness along with many of her undead comrades as the Lich King weakened during the events of the Warcraft III: The Frozen Throne expansion. The Lich King's energies were escaping from his icy prison; for though Illidan Stormrage's first attempt to destroy the Lich King had failed, the Frozen Throne had been fractured in the devastation. Sylvanas held a burning hatred for Arthas as a result of the part he played in destroying Quel'Thalas and trapping her in undeath as a 'monstrosity', subjecting her to eternal agony.

The Dark Lady ambushed Arthas as he was preparing to go to the aid of his master, the Lich King. She and her Banshee sisters set a trap for the fallen prince outside the ruins of Lordaeron City, but the timely rescue of Kel'Thuzad allowed Arthas to escape and continue his trek to Northrend. She ran off, swearing she would never stop hunting Arthas.

After the unsuccessful assassination attempt, Sylvanas turned her attention back toward lordship over the Plaguelands. She had coordinated attacks against Arthas' forces along with the Dread Lords Balnazzar, Detheroc and Varimathras, but that alliance quickly fell apart after the Death Knight's escape to Northrend. She captured Varimathras and, in exchange for his life, he pledged his allegiance to Sylvanas and assisted her in eliminating the other two nathrezim. Her rule over the Tirisfal Glades cemented, Sylvanas established an undead faction known as the Forsaken and completed the tunnels under Lordaeron City started by Arthas. This realm is now called the Undercity.

Currently the Forsaken's Royal Apothecary Society is engineering a new plague that will not only wipe out the remaining humans in Lordaeron, but the Scourge in Northrend as well.

Even in undeath, Sylvanas is considered to be the best archer on Azeroth. She claims that she can easily hit a bird in midflight straight in the eye. Though her proficiency in magic is strong, even being capable of performing mind control, she is more inclined to use her bow and arrows in combat. She prefers hit-and-run tactics to all-out assaults. However, as the leader of the Forsaken, she is now very unlikely to risk her own safety by leading an army.

Sylvanas is sister to Alleria Windrunner, hero of Warcraft II: Beyond The Dark Portal; and Vereesa Windrunner, hero of Day of the Dragon. Another Windrunner is Sylvos Windrunner, a Blood Mage hero. She has also been called "The Dark Rose of the Forsaken".

Source: Wikipedia.org

In pursuit of the Truth of the Dark Lady and her order of The Silvermoon Sisterhood, it is important to note that not all clues are written. Many may be discovered by close observation of dress, expression, and behavior. For example, as any loyal subject knows, who has spent time at the feet of the normally austere Dark Lady of the Forsaken, she has a silly, impish side. If one waits patiently, every five minutes or so, after a careful look around, she may be observed breaking into a smile and jumping up and down like the elven girl she remains beneath her stern, deadly majesty.