Jaqen H'ghar from A Clash of Kings is Arya's saving grace. In an act of compassion, Arya frees Jaquen and othe other prisoners when they would surely have died of a fire. In return, Jaquen tells her that since she spared three lives, three lives must be given in return. She must decide who will die. Arya is happy to hear this because she has a long list of people that she recites to herself before going to bed at night. She has him kill two people fairly fast, then cleverly tells Jaqen that he is the last one to die. Understanding that she has a keen wit, Jaquen says that instead of his own death he will kill the guards so that she can escape. He gives her a coin and tells her to give it to anyone from Bravvos and to say Valar Morghulis and they will help her.
