The Patriarch of Pestilence. The Milward Chronicles, #4

In a fit of desperate arrogance, Gilgafed the Sorcerer opens a rift between the worlds and releases a horror of old, a Seeker. Combined with the soul of a thief named McCabe, the creation becomes the one prophecies call The Destroyer. Aware now of his mistake and regretting his impulsive action, Gilgafed knows he must seek help to rid the world of this monstrous evil,  but the only one capable of aiding him is his ancient enemy, Milward, last of the Wizards.
Milward has his hands full with the conflicts raging through the Southern Empire. Charity and Adam, twin scions of the ancient House of Labad, have been reunited after being separated for years. They and their companions attempt to capture Bilardi, the mad Duke of Grisham, in order to stop a war that could decimate the countryside. Duke Bilardi, on the other hand, has become seduced by the same dark sorcery that has kept Mallien, Magister of the Grisham Church, in power.
With the rift weakening and ancient prophecies coming to fruition, Milward, the companions, and Gilgafed are forced to combine forces to survive--if they can keep from killing each other in the process.