The Mage Storms: A Valdemar Omnibus(1994)

Mercedes Lackey

Karse and Valdemar have long been enemy kingdoms, until they are forced into an uneasy alliance to defend their lands from the armies of Eastern Empire, which is ruled by a monarch whose magical tactics may be beyond any sorcery known to the Western kingdoms. Forced to combat this dire foe, the Companions of Valdemar may, at last, have to reveal secrets which they have kept hidden for centuries... even from their beloved Heralds.