Magic's Promise by Mercedes Lackey

Magic's Promise : Paperback : Penguin Putnam Inc : 9780886774011 : 0886774012 : 01 Jan 1994 : The wild magic is taking its toll on the land, and even Vanyel, the most powerful Herald-Mage to ever walk the world, is almost at the end of his strength. But when his Companion, Yfandes, receives a call for help from neighboring Lineas, both Herald-Mage and Companion are drawn into a holocaust of dark magic that could be the end of them both.