X-men #4

X-Men #4, published by Marvel in March 1964, marks a pivotal moment in mutant history. This issue features the first appearances of several key characters: Quicksilver, Scarlet Witch, Toad, and Mastermind. These new mutants join forces to form Magneto's original Brotherhood of Evil Mutants, who immediately challenge the X-Men. The story also includes the X-Men training in the Danger Room and celebrates their first-year anniversary as a team. Written by Stan Lee with art by Jack Kirby, this issue is highly notable for introducing these iconic characters.