This idea popped into my head yesterday after watching the Star Trek movie. It has nothing to do with the Star Trek movie; I think it came about from the combination of one of the preview trailers for another movie (cannot remember which), Final Fantasy Tactics Advance 2, Ah My Goddess (with Record of Lodoss War and Naruto thrown in), the song Just Dance by Lady Gaga, and other unidentifiable factors.
I want to go back and re-watch the episodes of Naruto with the flute-player demon ninja girl. I remember very little about her but she is definitely a bit of an inspiration for this.
Record of Lodoss War ties in because the idea has a semi-D&D fantasy feel to it but the techno dance mix of Just Dance wants a more modern setting.
Ah My Goddess plays into the cute, unassuming appearance of Black Dirge.
The set-up is that there is a heartless villain who leaves soulless, undead husks in his wake. He calls himself the Black Dirge and the magic of his music allows him to literally rip the soul from his victim's body. The soulless body then animates through the power of his music and wreaks havoc. If he is able to concentrate, Black Dirge can utilize the energy of the soul to empower himself or turn over the soul to the demon lord granting him power or forcefully morph the soul into an undead or demonic spirit alongside the animate, undead body. Whatever he does, it should be inhuman and diabolical. If he is distracted, the soul is left as a spirit to wander, unable to find rest (but able to be put to rest by the PCs or possibly even reunited to the body).
Black Dirge's power is great (ripping a soul from the body is hard work) but requires a lot of time and concentration. He operates like a serial killer, isolating victims, restraining them, and then working his magic. Horror should be the main point to get across; this man is inhuman, his acts demonic, and his next victim could be anyone, anywhere (he may or may not have a rhyme to his reason for choosing victims).
When the PCs finally encounter Black Dirge, he is tall, lithe, stark, sharp, and dresses like a death-obsessed freak. He dies his wild hair white and black, pales his face with cream, draws black streaks across his eyes with make-up, wears a cape with skull clasps, etc.
The secret here is that the real Black Dirge is a cute, sweet young woman with a sunny disposition who dresses in bright colors. The man is her apprentice and it was his choice to dress that way. She disapproves but allows him his eccentricities.
The real Black Dirge believes that her actions are beautiful and artistic. While he knows little about his victims and delights in the spread of death, she prefers to integrate herself into her victims' lives, truly delve into who they are, and finds a black, quiet beauty in their death. The thought is similar to the disgusting sense of "romance" one might find in a truly egregious snuff film - to be the first and last man to ever take a woman before her life is forfeit. The killer thus feels he "owns" that woman and her experience. No other man can share the experience he has had. Black Dirge enjoys knowing what and who a person was before she ensures that person no longer be. She absorbs the sense of loss and mourning.
Unfortunately, while Black Dirge prefers to spend lots of time with a single victim, her power far surpasses that of her apprentice. It took tremendous effort for him to rip the soul from a single victim. Black Dirge can kill a hundred people with a few notes. She prefers to take it slow but is capable of ending life in a flash.
I have yet to decide how closely Black Dirge will deal with demons. The cute face and sunny demeanor are meant to stand in stark contrast with her demonic actions. I want to avoid making her look psychotic or demonic like you would expect in anime once the jig is up (such as her iris shrinking down to a point or the blank all-eye iris usually indicative of mind control, sweat beaded on the forehead, a strained face with open mouth gasping in air or shouting, and a psycho laugh). Black Dirge has so separated from basic human decency that she can kill scores of human beings without it assailing her sanity.