duties of vassals

Mab smiled. Her teeth were small and white and perfect, her canines delicately sharp. "Alas. The Leanansidhe is tied up at the moment."
I drew in a breath. My godmother was a powerful member of the Winter Court, but she couldn't hold a candle to Mab. If Mab wanted to take Lea down, she certainly could do it-and for some reason the thought spurred on a protective instinct, something that made me irrationally angry. Yes, Lea was hardly a benevolent being in her own right. Yes, she'd tried to enslave me several times in the past several years. But for all of that, she was still my godmother, and the thought of something happening to her angered me. "For what reason have you detained her?"
"Because I do not tolerate challenges to my authority," she said. One pale hand drifted to the hilt of the knife at her belt. "Certain events had convinced your godmother that she was no longer bound by my word and will. She is now learning otherwise."
"What have you done to her?" I asked. Well. It didn't sound like a question so much as a demand.
Mab laughed, and the sound of it came out silvery and smoother than honey. The laugh bounded around the waves and the earth and the winds, clashing against itself in a manner that made the hairs on my neck stand up and my heart race with a sudden apprehension as I felt an odd kind of pressure settle over me, as if I were closed into a small room. I gritted my teeth and waited the laugh out, trying not to show how harshly it had affected me. "She is bound," Mab said. "She is in some discomfort. But she is in no danger from my hand. Once she acknowledges who rules Winter, she will be restored to her station. I can ill afford the loss of so potent a vassal."
"I need to speak to her now," I said.
"Of course," Mab said. "Yet she languishes in the process of enlightenment. Thus am I here to fulfill her obligation to teach and guide you."
I frowned. "You locked her away somewhere, but you're keeping her promises?"
Something cold and haughty flickered through Mab's eyes. "Promises must be kept," she murmured, and the words made wave, wind, and stone tremble. "My vassal's oaths and bargains are binding upon me, so long as I restrain her from fulfilling them."
"Does that mean that you will help me?" I asked.
"It means that I will give you what she might give you," Mab said, "and speak what knowledge she might have spoken to you were she here in flesh, rather than in proxy." She tilted her head slowly to one side. "You know, wizard, that I may speak no word that is untrue. Thus is my word given to you."


Dead Beat Chapter 21, Page 185-186