Uncanny X-Men Vol.1 #126
The X-Men return to Muir Island to investigate the attack on Polaris and Havok. When they get there they find themselves in a deadly battle against arguably the most powerful and deadliest mutant alive - Mutant X!
Printed: Oct 1979
Writers: Chris Clairemont and John Byrne
Art: John Byrne and Terry Austin
Cover: Dave Cockrum and Terry Austin