The High Lord: Book 3 of the Black Magician by Trudi Canavan
In the city of Imardin, where magic and power walk hand-in-hand, a young street-girl, adopted by the Magicians' Guild, finds herself at the centre of a terrible plot that may destroy the entire world ...Sonea has learned much at the Magicians' Guild and the other novices now treat her with a grudging respect. But she cannot forget what she witnessed in the High Lord Akkarin's underground chamber - or his warning that the realm's ancient enemy is growing in power once more. As Akkarin reveals more of his knowledge, Sonea does not know who to believe, or what she most fears. Could the truth really be as terrifying as the High Lord claims? Does an ancient power threaten them all? Or does the guild face a threat from a much closer source?