Maou-sama no Yuusha Toubatsuki

The Demon King rules over all demons, and using his immense power, put the human race through 1,000 years of oppression. Thus, the human's era has ended and the demon's era has begun. Lucifer has inherited the title of Demon King passed down from his father. However, rather than fight the humans, he much rather just get along with them. Not only that, he becomes sick at the sight of blood. Since all the humans knew where the Demon King's castle was, he was constantly trying to be killed by assassins. So he decided to leave the caste and live out in the wilderness where humans couldn't find him... or so he thought. Since every man with power has been defeated by him, the humans sent a girl to defeat him. How will Lucifer deal with this beauty sent to destroy him? (Source: MU)