Xavier confronts his nemesis, Lucifer, in his Balkan cavern. Lucifer has created a gigantic atomic bomb that will destroy the Earth if his heartbeat stops. The X-Men have been summoned to Europe to assist Xavier, if necessary. The Avengers have also come to Europe since Thor has detected evil impulses there through his uru hammer. The X-Men must stop the Avengers from assaulting Lucifer's lair for fear that it may result in Lucifer's death, hence setting off the bomb. (Cover Date: January, 1965)