Sylvanas Windrunner was once a High Elf Ranger-General in the service of the elves of Silvermoon and defender of Quel'Thalas. Through misfortune and dark magic, Sylvanas fell during Arthas' attack on Quel'Thalas to gain control of the magical Sunwell there. After the battle, she was resurrected as a banshee by Arthas for giving him such a tough battle. Later, Sylvanas would regain her freedom and become the queen of the Forsaken.
The Second War
During the second war, Sylvanas played a key role in pushing back the orcish assault within Quel'Thalas. Learning that the orcs were burning the area, she helped trap the horde between elf and alliance forces. After a long and uncertain battle, the orcs were defeated.
Arthas' Invasion of Quel'Thalas
After the scourge began to infect Azeroth, Arthas was sent to capture the Sunwell of Silvermoon to resurrect the fallen necromancer Kel'Thuzad. It was during this invasion that he encountered Sylvanas Windrunner. Sylvanas was the only line of defense separating Arthas from the elven city of Silvermoon and his ultimate goal of capturing the Sunwell. Although Sylvanas and her rangers put up a valiant fight, they were defeated by Arthas' scourge. Sylvanas tried to send several elven runners to warn Silvermoon, but Arthas cut each one of them down, leaving the city completely surprised by his attack.
Sylvanas last living words to Arthas were that she deserved the peace of death, but Arthas, angry that Sylvanas had made him fight so hard to get where was stated, "After all you've put me through, woman, the last thing I'll give you is the peace of death." In a final act of dark evil, Arthas slew Sylvanas and raised her captured soul as a banshee. With Sylvanas under his power, Arthas was able to use her to help bring about the fall of Silvermoon and capture the Sunwell. Sylvanas was powerless to do anything other than help Arthas destroy her beloved homeland.
Sylvanas was forced to serve Arthas as a mindless slave for some time. It wasn't until the attack on Ner'zhul's Frozen Throne in Northrend by Illidan Stormrage and Lady Vashj that Sylvanas was able to break free of Arthas' control. The Frozen Throne was cracked and Ner'zhul's power weakened as it seeped through. This allowed Sylvanas to break free of Arthas' control without his knowledge. A mistake that nearly proved fatal for Arthas.
When Arthas returned to Lordaeron, he sent the three dreadlords that guarded the kingdom fleeing. Sylvanas, continued to serve Arthas as a deceptive ploy, not allowing him to know that she had been freed from his control. She would later plot with the three dreadlords to set a trap for Arthas so that she could take her vengeance on him. The attack on Lordaeron sent Arthas fleeing right into Sylvanas' trap. Three of her banshees awaited Arthas as a ploy to lead him to safety. Arthas followed the three to an ambush where Sylvanas awaited. When Arthas arrived he was shot with a paralyzing arrow and left at the mercy of Sylvanas. Arthas was furious at the betrayal and demanded she just finish him, but Sylvanas was not content with giving Arthas the peace of death that she did not receive. A mistake she soon paid for as the necromancer Kel'Thuzad intervened and stopped Sylvanas from taking her vengeance.
Queen of the Forsaken
Sylvanas would later return to physical form as one of the undead. Knowing that she would never be accepted by the living world she once knew, Sylvanas formed a dark version of the rangers she once led known as The Dark Rangers. After her attempt at killing Arthas failed, she was approached by the dreadlord Varimathras once again. He tried to recruit Sylvanas and her newly freed minions into their service, but Sylvanas was not so quick to surrender her new freedom and pledge allegiance to the dreadlords. Varimathras was not pleased and as he departed, Sylvanas knew that an attack would soon follow. With only a few supporting her, Sylvanas knew that she would not last without more help. However, with the help of her banshees, Sylvanas was able to possess the leadership of several factions within the Plaguelands and form a formidable army.
Varimathras tasted defeat when he next met Sylvanas. The dreadlord begged for his life, vowing to serve Sylvanas. Sylvanas knew that the dreadlord could not be trusted, but feeling she could use him, she spared his life. Varimathras would later aid in the attack and defeat of another dreadlord, Detheroc. As Sylvanas defeated her enemies within the Plaguelands, she announced that they would rule the Plaguelands as the Forsaken. Lady Sylvanas now rules over the Forsaken from the Royal Quarter of the Undercity.