The Necromancer Trilogy - Black Magic

Sophie Merith is going to destroy the world. The only way she can escape her fate? A stone from the lands of the dead and two obstinate boys whose duty is purely to the souls of the departed.

With the Gatekeepers found, the countdown to Midwinter begins. To protect the Yale brothers, Sophie and her compatriots must risk their lives to thwart the devotees of the most powerful man alive.
But as the season gets darker, Lord Legion grows ever more dangerous. There is battle approaching, battle in which all sacrifices must be made.

Third book in the trilogy.