Windrose Chronicles, book 3: Dog wizard
Barbara Hambly1993

The Citadel of Wizards was under siege. Deep in the Vaults beneath the walled city, Gates into inhuman realities were opening and closing without warning. Monsters emerged from other dimensions; and ancient, malign magics came to life under the influence of cosmic chaos.

Someone was tampering with the Void.

The Council of Wizards blamed Antryg Windrose, the rebel mage who had fled their justice to find haven in the City of Angels. Across dimensions, they reached out to bring him back...

Antryg refused the wizards' summons. But when Joanna was kidnapped, Antryg followed the woman he loved back across the Void. Trapped in the Citadel between the vengeful mages who once had been his friends and the unspeakable horrors in the Vaults below, Antryg launched a desperate bid to rescue Joanna -- while all about him, the very fabric of the universe unraveled.