Wizards at War book by 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 of Earth and many other planets find themselves forced to defend wizards and nonwizards alike against an invasion of a kind they've never imagined.
But mere defense won't be enough to combat the evil afoot. With their alien teammates, Nita, Kit, and Dairine must race to search worlds known and unknown for the secret weapon the Powers That Be have promised them--before the minions of the sinister Lone Power find it first. And then, for the first time in millenia, the wizards must go to war. . . .