Xenu (also Xemu) (pronounced /ˈziːnuː/ or /ˈziːmuː/), 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 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. 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 about 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."