The evil galactic overlord that stupid, stupid, gullible people known as "scientologists" believe to have wiped out billions of aliens on Earth 75 million years ago. Then Xenu collected their souls, known as 'Thetans', subjected them to brainwashing (how ironic) before releasing them back into the world, where they possessed our bodies and caused all of our problems.
"Check check check it holmes, I'm gunna pimp-slap Lord Xenu an' his Thetans by givin' mah life's savings to some crooks, aiiiiiight! That shit be TIGHT!!! Then I'm going to live in a cardboard box under a bridge."