Wings Of Wrath: The Magister Trilogy: Book Two by Celia Friedman
In a world where the price of magic is life itself, a group of seemingly immortal sorcerers appears to have cheated the system. But only one man knows the true origin of their power, or understands the true cost. Now Kamala - born to poverty and abuse, the first woman to claim a Magister's power - will seek her rightful place among these mages, and lay siege to their secrets. The monk Salvator will claim his father's throne, and test his faith against a legendary darkness. The beautiful Siderea Aminestas, consort to Magisters, will be offered the thing she desires most - at the cost of her human soul. And an ancient Evil thought long-destroyed begins to stir anew, corrupting kings, shattering alliances and ultimately threatening to unweave the very fabric of human civilisation. A mystical bloodline was cultivated to withstand this darkness, and its power must be wakened. But this will demand sacrifice of its warriors - and corruption is rife.