18+ Ishi no Sekai no Zecchou Kenshi

In a region far from the frontlines of the battle between humanity and the Demon King, an incident occurred where an entire town was petrified. The Adventurers' Guild decided to dispatch two highly skilled individuals to investigate the cause: a saint elder sister and a dual-sword wielding younger sister. The younger sister, Rui, possesses a skill where her sensitivity increases the more damage she receives. One day, after sustaining a fatal wound, her overly sharpened sensitivity goes berserk, and despite being on the verge of death, she is assaulted by unimaginable pleasure...

(Source: Akita Shoten, translated)