Let me tell you a little bit about the Forsaken before we get to Lilian Voss, for those who never played WoW.
Originally Forsaken were those undead that became free of the Lich King’s control, so technically they were all former scourge killed by him, or the plague. Lady Sylvanas Windrunner is the leader of the Forsaken, which honestly is a little odd because she is formerly an elf, and all forsaken are human, but it makes sense if you know the story.
When all of the scourge kind of “woke up” after being released from the Lich King’s mind control, they were tormented by the memories of all they had done under his command and were basically universally hated and mistrusted by all the races on Azeroth, who sought their destruction. Sylvanas was a beacon of light in the darkness. She gave them leadership, focus and hope. Her burning desire for revenge against the Lich king gave them a new purpose. They took over what is now called the Undercity, which were catacombs under the ruined city of Lordaeron. Prince Arthas’ home. Looking for allies to protect her new people Sylvanas reached out to the Tauren whose leader saw in them a chance for redemption and talked Thrall (leader of the Horde at that time) into an alliance against the Lich King.
Here’s where we come to Lilian Voss. After the Lich King’s defeat there were no more scourge created and therefore no “newborn” forsaken. With the knowledge that eventually her people would die out since they had no way to reproduce, Sylvanas made a deal with the Valkyr who began to “awaken” new undead for her. Lilian Voss was one such undead. The special thing about Lilian Voss is, she was a member of the Scarlet Crusade in life….To refresh your memory the Scarlet Crusade is a fanatic organization created with one purpose. To destroy all undead. When she was risen by Sylvanas’ Val'kyr she was horrified at what she had become and ran away to find her Father, hoping he would know what she could do, and help her.
Before she could get to him however she was captured by a group of the Scarlet Crusade and imprisoned, after she told them who she was. Her captors then sent word to her father, High Priest Benedictus Voss, who ordered her immediate execution when he discovered what had happened to her. As you can imagine this did not go over well with Lilian. In an astonishing display of power she exploded in purple flames and lept through the bars of her cage, incinerating her captors while remaining unharmed.
Lilian Voss says “the world of the living may have turned it’s back on me, but I’m no damned Scourge.”
Betrayed by her people and possibly now seeing the Crusade for what it was, callous and uncaring of who it killed, Lilian turned her murderous skills on the very people who had trained her. A bounty was put on her head and the alarm was sent to all Scarlet Crusade organizations across the Plaguelands but she knew their tactics too well and left bodies wreathed in violet flame all across Tirisfal Glades.
(To the Scarlet Crusade, anyone and everyone who wasn't a member of the cause was a potential carrier of the plague or an undead in disguise. After all, nobody in their right mind would idly wander through the plaguelands unless they were somehow in league with the Scourge.)
You meet Lilian in the game as she plans the death of her father.
"My name is Lilian Voss. Before I died, I was a member of the Scarlet Crusade. My father, High Priest Voss, raised me to be a weapon against the plagued. I studied stealth, sorcery, martial arts, anything to make myself stronger. I gave up my childhood for him.
Then, as fate would have it, I died and returned as... this. My father instantly forgot me and, when I returned, ordered me executed.
Come. We will speak with him now, in his tower to the northwest."
The trademark of Lilian Voss is that the bodies of her victims are always found wreathed in purple flames. In Legion she is a Champion of the Uncrowned which is a secret order of rogues who claim to be “The slayers of kings, the downfall of empires, the unseen blades that write the true history of this world” Another notable member is Valeera Sanguinar.