The High Lord (Black Magician Trilogy)

Life should have been easier for Sonea, now that Regin ignored her and the rest of the novices treated her with wary respect, but she cannot forget what she witnessed in Akkarin's underground room, or his warning that Kyralia's ancient enemy is watching the Guild closely. The last few years have been good for Cery. He has a respected position among the Thieves, and connections in high and low places. When an exotic stranger offers to help him with a secret task, Cery knows he should refuse, but life would be boring without the occasional risk... As Akkarin reveals more, Sonea does not know what to believe or what she fears most - that the truth is as terrifying as he claims or that he is trying to make her his accomplice in black magic.