"You are the only one left who can use the Breath of All Things, and Salazar, as the patient, could hardly cast spells during the process. I will be responsible for suppressing his continuously draining soul and restoring it to the most normal state possible, while Professor Snape will handle the spellcasting."
Powerful though the Breath of All Things was, it dealt with the human soul—the most mysterious and precious part of a person. Without Iblis's intervention, not even Dumbledore could attempt it lightly. Furthermore, repairing a soul required repeated practice and a deep comprehension of charms, placing immense demands on the caster's natural talent and magical power.
Iblis pointed to a thick stack of books on the desk. "In exchange, I will assist you in researching potions to repair the human soul, though that merely means providing experimental subjects. Along with all the ancient books preserved in the ring, you may freely consult and read them during the holiday. You won't find these in Knockturn Alley."
Chapter 73