From MangaHelpers:

This manga is based on the erotic game of the same name, which is part of the 'Koihime' franchise of erotic games. The main character, Nitta Kensuke, touches a sword in a storeroom one day and falls unconscious. When he wakes up, he is in the Sengoku ("warring states") period of Japan, except all the historical figures are sexy young girls and the country is under attack by human-eating, blood-drinking demons! [tethysdust]