She was a non-playable boss in Warcraft III: Reign of Chaos who then just fades into the background when you kill her as Arthas and then raise her as the first Banshee.
Come the expansion, Warcraft III: The Frozen Throne, Sylvanas not only has recovered her body (as in, she possessed her own corpse, being a Banshee and all), she has regained her free will due to the weakening of the Lich King, has begun consorting with Dreadlords, and was a literal arrow away from killing Arthas, who only managed to escape because Kel’thuzad saved his ass. She then goes to destroy the last Human resistance in the area and becomes the leader of the Undead that have freed themselves from the Lich King’s mind control.
The final battle of Charum Hakkor had broken
the human-San’Shyuum alliance and destroyed the last reserves of human
resistance. That battle had been notorious, a great victory - but from the point of
view of Mantle orthodoxy, of course, supremely disgraceful.
In an apocalyptic future machines have taken over the world. The last remains of the human resistance, the Rising Tide, send their best warrior Grant Ward who may or may not be a cyborg back in time to protect a young woman named Daisy “Skye” Johnson - the best hacker in the world and the only one capable of shutting down the machines and preventing the war from happening. But the machines and their deathly human-looking terminators are close on their heels…