1. The Mongolian Conquests,1207-1472, 30-60 million
Stretched out over more than 200 years, the Mongolian war machine crushed and enveloped all of Asia, the Middle East, and much of Europe. Most famously under Genghis Khan, this immense empire established itself through a bloodshed never before seen. While the death toll might not seem as high as WWII, the lower global population level meant that almost 17% of the world's people were killed during this incredible power push. It was an incredible death toll, and short of blanketing the Earth in nuclear bombs, we'll never see its like again. Thank god.