Uncanny X-Men #21

Whilst the X-Men seek a way into Lucifer`s desert citadel, Professor X is trapped by the villain`s robots.The team attack Lucifer`s headquarters and must battle whirlpools and other dangerous defenses to reach the interior. Meanwhile, Professor X talks to Lucifer and discover that he is actually an alien, send to Earth ten years ago to dominate it. He would do it with the help of a giant computer machine, called Dominus, that is operate by robots.After fighting their way through the labyrinth-like hideout, the X-Men finally confront the robots and defeat them. After knowing about Lucifer's defeat to the X-Men, the Supreme One banishes him to an eternal exiles and end the Earth invasion.