From amazon.com:

The third thrilling Chronicles of the Invaders adventure from New York Times best-selling author John Connolly and Jennifer Ridyard mixes classic sci-fi with rich, character-driven plot as Paul and Syl fight to save Earth from an enemy who is closer than ever.

Syl Hellais and Paul Kerr have traveled through Derith, the mysterious wormhole from which no traveler has ever returned. Yet Derith's secrets are darker than they imagined, and, trapped in a dimension beyond their own, they finally emerge to discover a universe that has moved on without them.

Years have passed, and civil war rages among the Illyri. It is whispered that Earth is lost, prey to the alien parasites known as the Others, and other worlds will soon follow. Most shocking of all, the sinister Archmage Syrene of the Nairene Sisterhood has disappeared into the bowels of the Sisterhood's lair.

But before she cloistered herself, Archmage Syrene chose her replacement. The Sisterhood has a new leader with her own plans for the future of her race. Now Syl and Paul, teenagers in a deadly adult world, must find a way to change the course of history and save the lives of billions. They have but one hope.

For Syl Hellais is changing....