X-men: Age Of X - Mike Carey

In a world where the X-Men never existed and mutantkind has been hunted to extinction, the few remaining mutants have banded together to make their last stand. They spend each day securing the walls of their stronghold, Fortress X, beating back the anti-mutant forces that would see them wiped out. But when a conspiracy is discovered within Fortress X itself, could it be that mutantkind's last bastion will crumble from the inside out? Collecting: Age of X: Alpha; X-Men Legacy 245-247; New Mutants (2009) 22-24; Age of X Universe 1-2