Wizards at War — Diane Duane

Nita and Kit return from their wizardly holiday looking forward to getting back to their everyday routine. But there's trouble brewing. A strange darkness of the mind and heart is about to befall the older wizards of the world, stealing away their power. Soon, the young wizards find themselves forced to defend wizards and nonwizards alike against an invasion of a kind they've never imagined.
For the first time in millenia, the wizards must go to war. . . .