Storm Rising — Mercedes Lackey

The Dire Threat of War
with the powerful, sorcerous, Eastern Empire has forced the kingdom of Valdemar into an uneasy coalition with its traditional enemy, the neighboring kingdom of Karse.

But now, mysterious mage-storms are wreaking havoc on both Valdemar and Karse, plaguing these lands not only with disastrous earthquakes, monsoons, and ice storms, but also with venomous magical constructs -- terrifying creatures out of nightmare.

As Valdemar's Heralds and Karse's Sun-Priests struggle to marshal their combined magical resources to protect their realms from these devastating, spell-fueled onslaughts, the still fragile alliance between these long-hostile lands begins to fray. Only the personal intervention of Solaris, the High Priestess and ruler of Karse, can defuse what is rapidly becoming a dangerously explosive situation.

But Solaris also confirms the worst fears of the Heralds -- that these storms come from a mysterious, unknown source. And unless Valdemar and Karse can locate and destroy the elusive and enigmatic cause of these storms, they will see their entire world demolished in a final magical holocaust.