Xenu (also Xemu), , according to Scientology founder (and speculative fiction writer) L. Ron Hubbard, was the dictator of the "Galactic Confederacy" who, 75 million years ago, brought billions Thousands of millions in Long Scale of his people to Earth in DC-8-like spacecraft, stacked them around volcanoes and killed them using hydrogen bombs. Scientology holds that their essences remained, and that they form around people in modern times, causing them spiritual harm. Scott, Michael Dennis (2004) "Internet And Technology Law Desk Reference", Aspen Publishers, ISBN 0735547432 Lewis James R (2003) "The Oxford Handbook of New Religious Movements", Oxford University Press, ISBN 0195149866 Members of the Church of Scientology widely deny or try to hide the Xenu story.

These events are known within Scientology as "Incident II", and the traumatic memories associated with them as The Wall of Fire. The story of Xenu is part of Scientologist teachings on extraterrestrial civilizations and alien interventions in Earthly events, collectively described as space opera by Hubbard. Hubbard detailed the story in Operating Thetan level III (OT III) in 1967, warning that this material was "calculated to kill (by pneumonia etc) anyone who attempts to solve it." [ sic ]

Criticism of the Church of Scientology often includes details of the Xenu story. The Church has tried to keep Xenu confidential but critics say the story should be made public, given the high prices charged for OT III, part of Scientology's secret "Advanced Technology" doctrines taught only to members who have already contributed large amounts of money to the organization. The Church avoids making mention of Xenu in public statements and has gone to considerable effort to maintain the story's confidentiality, including legal action on the grounds of both copyright and trade secrecy. Despite this, much material on Xenu has leaked to the public, largely via the Internet.