New X-Men by Grant Morrison Vol. 3: New Worlds

As protesters lay siege to the Xavier Institute, mutant mentor Professor X lies in a coma, trapped within the shattered form of his evil twin! Cassandra Nova inhabited Xavier's body and escaped to the starts in the wake of her initial encounter with the X-Men; now, the psionically-empowered psychopath is returning to Earth- possessed of the most powerful mutant mind on the planet, and with the might of the Shi'ar Imperium at her back!