Humans and monsters shared the Continent in a delicate balance for over a thousand years, but it all came crashing down.

The most powerful fiend, Kayblis, seized power over the Monster Realm and led a colossal army on an invasion of the Human Realm. Unable to stop quarreling amongst themselves, the human nations were trampled.

Two weeks after the war began, the death toll for humanity reached approximately 12,000,000.

Yet mankind still couldn't unite, and the turmoil between the world's leaders continued. Just as the demise of humanity, the worst-case scenario, weighed heavily on everyone's minds...

"Everyone should just work for me. I'll crush that Monster Army no problem."

It was Rance, back from an adventure with his companion, Sill Plain.

There was an uproar among the world leaders:

The Kingdom of Leazas's Queen, Lia Parapara Leazas.
The Kingdom of Zeth's Viceroy, Magic the Gandhi.
The Republic of Helman's President, Sheila Helman.
The Free City Alliance's Representative, Copandon Dott.
The eastern island of Nippon's Lord, Oda Kou.
The world's largest religion, Alicism's Pope, Crook Mofus.

...And they were also Rance's women, so they all agreed. Rance became humanity's supreme leader and their final hope.

Over 6000 years, a minuscule creature called Kayblis fought and slaughtered his way to the very top of the Monster Realm. A conceited man named Rance, who declares all women to be his, lives a carefree life and finds his way to the top of the Human Realm. Both men put the survival of their entire race on the line in an inescapable struggle to the death.

[From MangaGamer]