Mirror Dreams Series by Catherine Webb

Every dream you've ever had, and every dream yet to come, exists in the Kingdoms of the Void. Every nightmare, too. Because there has to be balance; it's the rules. But the Lords of Nightkeep aren't big on rules; only Conquest, Fear, and Eternal Darkness for All. It takes a powerful wizard like Laenan Kite to keep them in check. But Kite has other worries, and Nightkeep is growing strong. Its Lords hunger for power. And they've turned their gaze towards earth.

Deadly spells are igniting across Haven, and the Queen of Dreams is in mortal danger. The court is pointing accusing fingers at Lord Rylam. One thing is clear—Haven needs Laenan Kite. He's the most powerful magician in all of Dreams, the bane of Nightkeep, the protector of Earth's sleeping souls. Queen Lisana wants him on the case fast, because if Rylam isn't the enemy, then the Realm of Dreams is in deep trouble.