The Beast Must Die

After finding out his sister has been gang-raped by a secret fraternity before she commited suicide, Lee Kirin decides to catch the rapists and take revenge on them. He approaches one of the known members of the group, Kang Moo, and asks him for his cooperation. However, in order to join the fraternity and find out who led his sister to her death, he is asked to have sex with everyone in the fraternity. As time goes by, this isn't the only challenge he has to face in order to come closer to the truth. (Source: Lezhin US) Note: Includes 39 extra chapters.