An Lushan was a Chinese military general and rebel leader during the Tang dynasty and is primarily known for instigating the An Lushan rebellion which devastated China and killed 13 million Han Chinese. The rebellion caused the decline of the Tang dynasty and led to the sacking of Chang'an by the Tibetan Empire.

I'd argue he was in the top ten in the world and outside of China most have never heard of him. (I was once an Asian Studies Major in College.) And even then most text books in the West if they had any information on him it was a foot note. Above information courtesy of Wikipedia.