(credit goes to Blizard for this character, and for the game its from, World of Warcraft)
Sylvanas was a member of the prominent Windrunner family of the high elves. Her siblings were Alleria, Vereesa, and at least two other brothers.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.
The Second War
During the Second War, the high elves initially sent only token support to the Alliance, though Sylvanas' older sister, Alleria, took her squad of rangers as well. Soon after, the forests of Quel'Thalas mysteriously began to burn. Sylvanas and her rangers set out to discover the cause when she came across both her sisters being chased by a band of forest trolls, whom they dispatched quickly. Alleria alerted Sylvanas of the coming orcish Horde, and that it was they who were burning Quel'Thalas with dragon fire. Sylvanas and her rangers moved quickly to meet the Horde and trap them between themselves and the Alliance forces under the command of the paladin, Turalyon. The battle took many turns but ultimately the Horde abandoned Quel'Thalas. Sylvanas stayed behind to hunt down any remaining orcs. Shortly after, the Horde was defeated, the Dark Portal was destroyed, and the Second War had ended.
Invasion of the undead scourge
After the high elves withdrew from the Alliance, Sylvanas and her Rangers were the primary guard against any would-be attackers on Silvermoon. Despite the odd forest troll, murloc, or gnoll every now and then, Sylvanas saw little action, and a peaceful tranquillity had descended upon the forests. She remained, however, ever vigilant, knowing that this peace— however refreshing —was tenuous at best.
Her fears were soon realized when Arthas, the betrayer of Lordaeron, arrived unexpectedly on the doorstep of Quel'Thalas with a horde of undead at his back. He began to attack outlying villages and immediately, Sylvanas and her Ranger Corps moved against him. His superior numbers and tireless warriors always kept her in a state of retreat.
Arthas continued relentlessly towards the Elfgate, killing every elf who got in his way. Finally, despite Sylvanas' best efforts, he breached the Outer Elfgate. Sylvanas ordered the corps to immediately fall back to the Inner Gate, which was only passable with the use of the Key of the Three Moons, assembled from the Mooncrystals hidden within the forest. Sylvanas destroyed the bridge leading to the Inner Gate to stay his advances. Despite her valiant efforts, Arthas managed to recover the Key of the Three Moons and destroyed the Inner Elfgate. Nothing stood in his way to Silvermoon.
Sylvanas gathered her forces and went to warn Silvermoon, but Arthas blocked her passage at every turn, and murdered every runner she tried to get through. Sylvanas continued the resistance, but eventually, she was targeted directly. The two battled each other, and Sylvanas was struck a mortal blow.
The mindless banshee
The official story given by the high elves of Silvermoon had the Ranger-General having perished valiantly in battle against the undead Scourge in the defense of Quel'Thalas, and her body being burned to ashes in the fire that had devastated half the capital.
The true story was quite different. Sylvanas was not killed, but rather captured. Barely clinging to life, Sylvanas Windrunner was brought before Arthas in his dark temple. She protested, saying she deserved a clean death, but Arthas, having had to fight her every step of the way, responded by telling her after what she had put him through the last thing he would give her was the peace of death. She was tortured, mutilated, and finally killed for Arthas' pleasure. He desecrated her spirit, corrupting her body and soul, and brought her back as a whirlwind of torment and hatred. Thus, Sylvanas Windrunner became the first of the high elven banshees. Sylvanas, now an undead banshee, was a complete and total slave of the Lich King. In her undeath, she aided Arthas in his assault against her own beloved Silvermoon in order to secure the legendary Sunwell for the Scourge. Her mangled body was sealed in an iron coffin by Arthas, as a way to further torture the ranger.
Sylvanas became one of Arthas' primary generals, and she remained behind in Lordaeron with Kel'Thuzad under the watchful eye of the dreadlords, whom had been left stationed in Lordaeron to protect it in the name of the Legion. When Archimonde was defeated at the Battle of Mount Hyjal, Sylvanas would quickly learn of this news from Kel'Thuzad. As the months passed, the dreadlords were still ignorant of their masters' defeat. Varimathras, Detheroc and Balnazzar, the three prime dreadlords overseeing Lordaeron, were convening under Sylvanas' undead watchful eyes when Arthas stormed into the stronghold, fresh from his journey to Kalimdor. He informed them of the Legion's failure and his intentions to take control of the Scourge. The dreadlords fled, outraged but unwilling to risk their lives by battling Arthas' army.
Arthas approached his servants, and ordered that Lordaeron be purged of all life as a tribute to Ner'zhul. Kel'Thuzad, however, informed him that human refugees had begun to flee from the outer villages, and that, should they escape into the mountain passes, they would be impossible to track. Arthas stationed himself and his two generals on the three possible escape routes and they each led detachments of undead troops against the fleeing humans, while taking great pains to ensure that few, if any, made it past their blockade to safety. Despite the efforts of the paladin Dagren the Orcslayer and his companions, Sylvanas led her banshees into battle against the human refugees, aided in the destruction of all the remaining villages in her area, and slaughtered any human trying to escape. Finally, Sylvanas, Arthas, and Kel'Thuzad arrived at the central headquarters of the paladins and killed them all in a vicious battle, annihilating the last of the civilian presence in Lordaeron.
The dark ranger
Around this point, the weakened Ner'zhul began to lose control of the minds of Sylvanas and a number of banshees. She kept this discovery from Arthas and Kel'Thuzad while continuing to serve. The dreadlords contacted Sylvanas and explained that they knew the cause, and arranged a secret meeting with her. They told her that Ner'zhul's— and by extension Arthas' —powers were beginning to diminish. The nathrezim intended to use this opportunity to seize control of Lordaeron. Sylvanas tentatively agreed to assist, but under the condition that her aid would be provided on her own terms.
The dreadlords planned on killing Arthas within the capital, but Sylvanas created a contingency plan in the case of Arthas' escape. She arranged for her banshees to feign allegiance and escort him to a place in the woods where she would be waiting for him. After Arthas survived the dreadlords' ambush and escaped the capital, Sylvanas' loyal sisters saw him to the arranged place, and then killed his bodyguards.
Sylvanas had, at this point, succeeded in using her abilities as a banshee to repossess the body that belonged to her in life. Sylvanas was now a corporeal undead creature. Hiding in the shadows, she struck him with an arrow she had crafted— a venomous sting that would paralyze him. Arthas was furious at her betrayal, and demanded that she finish him. But Sylvanas refused to show him the mercy of a quick death and intended to make him suffer— as he had made her suffer in undeath. She was about to begin her grisly work when Kel'Thuzad arrived and intervened, killing her banshees and chasing her off.
Back in her physical body, she realized that the natural world would never respond to her wishes again. Angered by this development, she turned to the arts that were becoming more and more natural to her: necromancy. She then altered her elven ranger teachings into a new form. Thus the dark rangers were born.
Queen of the Forsaken
Arthas left for Northrend, having been summoned by the Lich King, and Kel'Thuzad went into hiding. Sylvanas and her sisters were freed, but Sylvanas was still troubled. Though free from Ner'zhul, she and the other free-willed loyalists remained monstrous abominations in appearance.
Her thoughts were interrupted as Varimathras arrived on the scene. The dreadlord invited the banshee to join him and his brothers in their new order. But Sylvanas was not so quick to relinquish what freedom she had just gained. She told him that her assistance was enough, and demanded to be left alone. Varimathras ominously warned that those who were not part of this new land would be cast aside, and that she had best not vex the new rulers of the Plaguelands. Sylvanas was adamant. As Varimathras departed, Sylvanas knew that an attack was imminent. But with only her banshees and some lesser undead to serve for her, Sylvanas was certain that she would soon fall. She needed to acquire an army.
Scouting the outer wilderness, Sylvanas found a plethora of creatures who would serve her well, and with her banshees at her disposal, they would have little choice in the matter. She sent out her banshees to possess Mug'thol, the leader of the local ogres, the Bandit Lord, Blackthorn, the gnoll ruler, Snarlmane, and the Murloc Puddle Lord. Those she did not possess were killed. With so many new allies, Varimathras was quick to fall before her. But after she defeated him, Varimathras begged for his life, saying that he could serve her well. He knew his brothers' tactics, and where their bases were located. Sylvanas knew trusting such an insidious creature was a risk, but felt that she could control him enough for her purposes. With Varimathras' assistance, they moved against Detheroc's forces.
Detheroc had acquired a human puppet, Garithos, and his men, and was using them to protect himself. Sylvanas had his scouts possessed, and then infiltrated his base while their guard was down. As they slept, Sylvanas swept through the twin bases and slaughtered any before her. Though they eventually awoke and raised the alarm, it was too late. Sylvanas disabled the humans and ravaged Detheroc's forces, eventually coming to the dreadlord himself, and killing him swiftly. With the dreadlord dead, Garithos was freed from his mental control. Sylvanas saw him as arrogant and foolish, but felt that she could use him. She lied and promised to relinquish control of the Capital to him if he helped her kill Balnazzar.
Garithos attacked from the rear entrance to the city as Sylvanas and Varimathras came from the front. As they moved into the city, Sylvanas found caches of items left by the Lordaeron royals, and used them to further the assault. Despite the dreadlord's demonic allies, his forces were destroyed, and, as a final test of loyalty, Sylvanas demanded that Varimathras kill Balnazzar. Varimathras froze, saying that it was forbidden for one Nathrezim to kill another. But she required Varimathras to prove his loyalty to her. Varimathras obeyed, and struck down Balnazzar. However, whether by accident or design, Balnazzar survived, later re-emerging as a leader of the Scarlet Crusade. Varimathras was not so reluctant when Sylvanas ordered him to do the same to Garithos.
With all their enemies dead, Sylvanas made a proclamation to Varimathras. They would rule the Plaguelands as the Forsaken, and would stake out their own destiny, where none would get in their way.
Interestingly, reports say Sylvanas moves to and from Northrend; it's unknown if she is scouting for a possible attack on the Lich King, or if she has darker plans. There are rumors that Lady Sylvanas Windrunner is setting up a Forsaken city up there somewhere. In any case, she remains an enigmatic figure— her motivations unknown even to her most trusted advisers.