Lilian Voss

Lilian Voss is a recently "recovered" undead, first uncovered by the player from a coffin inside the Shadow Grave of Deathknell. The player, while leveling in Tirisfal Glades, will often either encounter or hear about Lilian up until the player helps her kill High Priest Benedictus Voss, her father.

It's unknown what happens to Lilian after those events, but the Forsaken would love to have her fighting for them.

Before her death
From the moment of her birth she was raised as a weapon against the plague as a member of the Scarlet Crusade. She studied stealth, sorcery, martial arts and anything else that would make herself stronger. She gave up her childhood for her father.

Tirisfal Glades
Lilian Voss was awakened by Sylvanas' val'kyr, finding herself utterly, completely horrifying. Her entire life had been devoted to the eradication of the undead - and after her death, she found herself in the position of becoming the very thing she'd dedicated her life to wiping out. To the Scarlet Crusade, the undead are an unholy abomination. Every moment of Lilian's life to that date had been filled with the doctrine and dogma of the Scarlet Crusade. When Horde players approach her to recruit her into the Forsaken's ranks, she runs away in denial of her undeath claiming that her father will protect her. She instantly cries for her father and runs away screaming for her life, pleading to be taken back home. Later she is found by Novice Elreth in Deathknell. To show her what she really is and to join the Forsaken Novice Elreth gives the player a hand mirror. After she sees her face Lilian runs away again calling for her father and denying her life as an undead.

Faced with her own horrifying reflection, Lilian fled, unwilling to accept or understand what had become of her. She sought out her father, a high priest of the Scarlet Crusade named Benedictus Voss. But before she could find her father, Lilian was captured by a group of Scarlet Crusade nearby. Though she told them who she was, the soldiers had no idea what to do with the young woman and kept her locked away while news of her fate was sent to the high priest. High Priest Voss knew exactly what to do when confronted with news of his undead daughter. He ordered her immediate execution. In an astonishing display of power, she burst into violet flame, leapt through the bars of the cage that held her and incinerated her captor. Confronted with the realization that she would find no comfort in the arms of her father, Lilian began a killing spree, enraged at the actions of her former comrades and leaving a trail of violet flame behind her.

All across Tirisfal Glades, the alert was raised by the Scarlet Crusade: The High Priest's daughter was a menace that needed to be destroyed, quickly. Having come to the inevitable conclusion that there was no going back, Lilian approached High Executor Derrington at the Bulwark in Tirisfal. While she didn't swear fealty to the Forsaken, she was quick to swear her enmity against the Scarlet Crusade. The High Executor appreciated her odd eagerness for bloodshed and sent her to take care of one of the few remaining Scarlet outposts to the north. A trail of violet-flamed bodies followed in her wake as she relentlessly continued to eradicate those who betrayed her. As Lilian made her way to a tower she mechanically ripped apart any Scarlet who stood in her way until she confronted her father. Scaling the walls of his tower, she strangled the life from her father, then threw his corpse to the ground with nary a word. And after it was over, Lilian stepped from the tower and vanished.

Nobody has seen Lilian Voss since that day.

Western Plaguelands
Later High Executor Derrington at the Bulwark receives reports of Scarlet Crusade activity at a small camp east of Felstone Field now known as the Charred Outpost. He sends players to the Western Plaguelands to inspect the encroaching Scarlets, but when you reach their camp they are all dead, engulfed in purple flames.

Quotes

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 * Lilian jumps on the Scarlet Bodyguard and, killing them instantly.
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 * Lilian grabs Benedictus and climbs up the walls of the towers, strangling and killing him in the process.

Speculation
She may be a reference to Liliana Vess from Magic: The Gathering.