The concept of Rightful Acquisition is an important part of Shranish culture. Rightful Acquisition is mostly limited to the acquiring of weapons. In old Shranish culture, if a warrior wanted a better weapon, he or she had to kill the owner to get it; in this way they were considered "worthy" of such a weapon, while the previous owner was clearly not. Weapons were never crafted on Shrane, and each warrior was expected to go to battle for the first time unarmed so as to be deserving of any weapon they acquired. By being "deserving" of the superior weapon, they believe they are in "rightful" possession of it. Through this process, the Shranish warriors believed that the highest quality weapons would naturally end up with the warriors of highest skill, eventually being paired with the warrior it meant to be with, as a soul would be paired with its intended mortal body.
This practice is precisely the reason Castus Render is believed to be from Shrane. He has a predilection for fighting without weapons; although he carries two shortswords, he almost never unsheathes them and he never takes new weapons. The reason is because he believes himself to be the superior weapon.