Star Wars - Knights of the Old Republic - Campaign Guide
Victim of the Mandalorian Wars, the man who became Nihilis lost everything: his possessions, his loved ones, and the will to live. While illness cannibalizes his body, a vast emptiness devours his humanity. Then, when all seems lost, he discovers that this emptyness hungered. The first time, he fed it unconsciously. Draining another being's life is frightening, nauseating, but for a euphoric moment, the memories, the illness, and the hunger disappeared. (Hey! Soul Eater from Nverwinter 2: Mask Of Betrayer)
There is more to the story, but I just cut it down for simplicity.