X-Men (1963) #126
"How Sharper Than A Serpent's Tooth" is written by Chris Claremont and drawn by John Byrne and Terry Austin. It begins with a panicked scream from Polaris broadcast over telephone lines to the X-Men Westchester, N.Y. mansion. Within minutes, Cyclops, Wolverine, Banshee, Nightcrawler, Storm and Colossus are winging their way across the Atlantic to Muir Island, off the coast of Scotland where Lorna Dane and Havok are helping Moira MacTaggert test Phoenix' power levels. Not knowing what danger they'll face, the X-Men prepared themselves for the worst.
Arriving on the stormy island, the mutants find the facility in disarray and Moira's guests either unconscious or beaten down. Because they thought the X-Men were dead, Havok, Polaris and Phoenix are very glad to see their teammates. But their joy is tempered by the knowledge that the ultra-powerful Mutant X has escaped Moira's containment cell and is now headed for the mainland of Scotland inhabiting the body of one of Jamie (the Multiple Man) Madrox's clone bodies. With the ability to warp reality, Mutant X or Proteus as he is called is more than a match for the X-Men who pursue him - turning Wolverine and Nightcrawler's reality into a nightmarish miasma of pain. Can Storm halt the murderous mutant's advance?