Written by JOSS WHEDON. Penciled by JOHN CASSADAY. Cover by JOHN CASSADAY. A tragic death at the Xavier Institute reveals a powerful enemy living among the X-Men that they could never have suspected - and no, it's not Magneto. Things heat up in a way none of the X-Men ever dreamed, but will teamwork save the day when they can't even depend on themselves? Collects ASTONISHING X-MEN #7-12. 144 PGS./Marvel T+