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.
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 maker herself stronger. She gave up her childhood for her father and in return, after she died and returned as an undead, he instantly forgot about her and ordered Lilian's execution.
She now seeks retribution against him and all of the Scarlet Crusade.
Although not a completely unique model, it's unusual for a non-death knight to have glowing blue eye sockets, while normally undead members (like the Forsaken) have glowing yellow eye sockets. It should also be noted that many of her attacks cause her victims to erupt in purple flames, leaving her mark throughout Scarlet encampments in Tirisfal Glades and the Western Plaguelands.