X-Men (1963) #6
"Sub-Mariner Joins the Evil Mutants" is written by Stan Lee and drawn by Jack Kirby and Chic Stone. Namor, Prince of Atlantis, is arguably the most powerful mutant in the Marvel Universe. Born to a human sea captain and an Atlantean princess, the Sub-Mariner can fly, has super strength and is virtually invulnerable. Sought out for his immense power, Sub-Mariner meets Magneto, the mutant master of magnetism, who fancies himself the mightiest mutant. Magneto hopes to build the power of his Brotherhood of Evil Mutants by adding Namor to its ranks. But Professor X and his X-Men hope to locate the Sub-Mariner before Magneto can recruit him.
Magneto finds the Sub-Mariner first, however, and in his astral state overhears the Atlantean prince ranting against his latest defeat at the hands of the Fantastic Four. Magneto plants a suggestion in the ear of a royal adviser that'll ensure the humanity-hating Namor joins his side. On a remote island, Magneto has built a devastating weapon which he uses when the X-Men track him down. Then the Sub-Mariner takes on the merry mutants himself! Read on to learn how Quicksilver and his lovely sister, the Scarlet Witch, change the balance of this battle. Plus, a special Cyclops pin up by Kirby.