Known as Sally in her life, Sally was a young human girl born into a family that hated Orcs! She was taught by her father to hate and kill all Orcs when she reached adulthood, and Sally did so! She killed Orc after Orc growing up, and she couldn't stop killing Orcs! She chopped their limbs off, cut off their heads, and enjoyed every second of it all! In time, the young, adventuring mass Orc-murdering maiden became so consumed by her insatiable lust to kill Orcs, she ended up dying. It would not end here however! Her corpse was later retrieved by the Scourge, and she was reborn a Death Knight. The desire to inflict pain caused by her undeath along with her obsession to kill Orcs caused her to essentially become a mindless monster, something that pleased Kel'thuzad in particular!
As a test to observe her thirst, Kel'thuzad placed Sally in an arena within Naxxramas, and presented to her many captive Orcs that the Scourge captured in their errands. Going into a hysterical mania, Sally cut down the Orcs in a shower of gore. Limbs flew about, heads were lobbed, intestines spilled, it was a horrific scene indeed! Kel'thuzad pleased with his new Death Knight, granted her the title Nihilus, and sent her off to the Military Quarter to complete her training.
Years later, Nihilus attended the Battle for Light's Hope Chapel, and due to her sheer insanity, and raving obsession to kill Orcs, she easily broke through the mental control the Lich King held over her. Uninterested with the fall of the Lich King, the wayward Nihilus initially went on a killing spree once more. Having the time of her life cutting up Orcs, it was cut shirt however, as the Kor'kron took notice and chased her off.
Having become bored of simply killing Orcs as she saw fit, Nihilus took up bounty hunting for the Alliance. Killing all kinds of enemies to the Alliance, Nihilus soon amassed a great fortune, but money did not interest her to a great degree. Gradually becoming unamused by bounty hunting, she threw most of her money out into the streets, and came the conclusion that only one thing truly made her happy. The sight of dead Orcs!
With that, she set out on her rampage once more! Though she was never heard from the Alliance ever again, and so it had been assumed in her zealous crusade she had died.