Sekai ni Hitotsu dake no R

In a world ruled by chaos and darkness lives a warrior whose family was killed by demons, his name is Urus. Driven by madness and revenge, one day he hears of "Kelg's pub", a place where heroes who were reincarnated into this world go to search for companions in their quest to defeat the demon king. Wanting to be chosen by one of these heroes, Urus makes his way to this pub but... There he is labeled as the weakest class by a strange summoning ritual and branded with the mysterious letter "R". It is here that he comes into contact with the true darkness of this world. What is "R"? What is this ritual!? And what is the darkness of this world!? (Source: Takeshobo, translated)