Millennium is a crossover event published in 1987.
The core storyline was an eight-issue miniseries written by Steve Englehart, with illustrations by Joe Staton. Every ongoing series being published at the time received crossover tie-ins to the event.
The story involves a Guardian and Zamaron visiting Earth to start the next wave of human evolution. Ten humans are chosen to advance and become immortals, given special powers. They are attacked by Manhunter agents, part of an alien conspiracy posing as close friends and relatives to every single superhero. Eventually this new breed of heroes become the New Guardians.