Power from bargains

"Lea. It's short for Leanandsidhe, isn't it."
“Yeah. I don't know her real name. She takes blood from mortals and gives them inspiration in return. Artists and poets and things. That's how she amassed most of her power."
Michael nodded. "I've heard of her. This bargain you have with her. What is it?"
I shook my head. "It isn't important."
Something shifted in Michael, became harder, more resolute. "It is important to me, Harry. Tell me."
I stared at the babies for a minute, before I said, "I was a kid. Things fell out with my old teacher, Justin. He sent a demon to kill me, and I went on the run. I made a bargain with Lea. Enough power to defeat Justin in exchange for my service to her. My loyalty."


Grave Peril Chapter 22, Page 224-225