Xenu, also called Xemu, is a figure in the Church of Scientology's secret "Advanced Technology", a sacred and esoteric teaching. According to the "Technology", Xenu was the extraterrestrial ruler of a "Galactic Confederacy" who brought billions of his people to Earth in DC-8-like spacecraft 75 million years ago, stacked them around volcanoes, and killed them with hydrogen bombs. Official Scientology scriptures hold that the thetans of these aliens adhere to humans, causing spiritual harm.
A DC-8 aircraft in 2004. Hubbard described Xenu's spacecraft as looking exactly like DC-8s without "fans" (meaning the jet engines, or turbofans).