The Silent Tower

Description for THE SILENT TOWER by Barbara Hambly:

LEGACY OF THE DARK MAGE

Someone was crossing the forbidding Void between the universes. In San Serano, Joanna felt the terror strike before dark hands were seizing her to kidnap her and take her...elsewhere. In the Empire of Ferryth, Caris watched a wizard murdered by a figure that vanished into a tunnel of darkness. And abominations were crossing through the weakening fabric of the Void.

And in the Silent Tower, where every stone was sealed and spelled against all magic, Antryg Windrose, student of the Dark Mage, was supposed to be going mad.

Only Antryg and the Dark Mage had understood the Void, the Archmage claimed. Yet the Dark Mage had been executed for his evil twenty-five years before. And Antryg was helpless. Or was he?

This is #1 in Windrose Chronicles.