Ixion killed his father-in-law and went mad from guilt at violating the Greek rule of "Xenia", which is a civilizing pact between host and guest. Zeus took pity on Ixion and invited him to Olympus. Ixion tried to seduce Hera (again violating Xenia), Zeus tricked him into having sex with a cloud, and of that union was born human looking Centaurus. Centaurus later had sex with a horse and thus the centaurs were created. Ixion was cast into Tartarus and nailed to a turning wagon wheel.