Regulus of Hogwarts: Lord of the Stars

"That is a very dangerous field," Sprout said slowly. "Sound-based magic — particularly the kind that touches the mind — sits at the boundary between advanced Dark and Light magic.

There have indeed been wizards who studied therapeutic harmonics throughout history, but most of those experiments ended in tragedy."

"Because it's too difficult to control?"

"Because sound is intangible and diffuse." Sprout explained. "You can guide the trajectory of a spell with precision, but controlling the propagation of sound is far more difficult.

Moreover, every individual's soul is subtly unique. A treatment that works for you could be poison for me. There is no universal solution."

'No,' Regulus thought. 'Sound can be focused. Physical means can achieve it — there's no reason magic can't.'

He filed the information away. No universal solution — but what if there were a way to scan individual souls? What if the harmonics could be custom-tailored?

And critically: if treatment has to be personalized, what about lethality?

'I could use it for wide-area killing.'