With the original X-Men on assignment (Marvel Team-Up #23), Professor Xavier turned to the Defenders for help. The result was one of the most important tales in mutant history (Defenders #15-16). The Defenders accompanied Professor X to the technologically equipped caves that Magneto and his latest incarnation of the Brotherhood of Evil Mutants were using as a base. The battle that transpired emphasized the strengths and weaknesses of each character.
- After learning that Mastermind's monsters weren't real, Dr. Strange still responded defensively to the convincing illusions, creating mystic mirror-images of himself to avoid getting hit.
- When punching alone didn't harm the Blob, Hulk was smart enough to uproot his grounded foe.
- Valkyrie's mystical blade could not penetrate the force field surrounding Unus the Untouchable, but she easily knocked Unus backward when he let down his guard.
- Nighthawk and Professor X were immobilized by the siren song of Lorelei, a woman Magneto had technologically mutated. But Xavier had enough willpower to reach over and activate Nighthawk's new jetpack, propelling Nighthawk into Lorelei.
Naïvely obeying Magneto's commands, Alpha levitated the U.N. building into the sky while Magneto proclaimed that it was his mutant birthright to rule humanity. When the heroes approached, Alpha used his vast power to freeze the Hulk in place, spin Nighthawk in a whirlwind until he blacked out, and turn Valkyrie's legs to putty. As Dr. Strange readied to cast a spell, Professor X intervened. Instead of fighting, Xaviar challenged Alpha to use his growing intellect to discern if Magneto was truthful or deceptive.
After contemplating the situation, Alpha set the U.N. building back in place and announced that he would leave Earth while his abilities continued to evolve. Before flying into space, Alpha punished the evil mutants by turning them into infants. The villains eventually recovered, but the event became a turning point in X-Men continuity. Magneto's rebirth accounted for his increased stamina and progressively sympathetic disposition in later years, and even came up during his landmark trial in Uncanny X-Men #200.