Jace, Isabelle, and Clary visit the Faerie Queen because they hope to make an alliance with her. Valentine intends to use the mortal sword to convert and enslave demon armies to his cause. In order to activate the sword he has to bathe it in the blood of four downworlders (including faeries). They try to convince her that the killings that have been taking place have not be renegade vampires, but rather the work of Valentine.
