This is the latest story of the famous "Ninpo-cho" series of Yamada Fuutarou. It begins in 1638, during the Shimabara Rebellion, where a powerful swordsman who should have died is instead transformed by an arcane ritual into a demonic creature. As time passes, more swordsman facing death go through this transformation. What could these new, demonic swordsmen be planning?

(Source: MangaHelpers)