Gu Zhen Ren
What is point of their existence? In this novel there is a lot of death and killing. Most people don't need to hide deaths. With that in mind there shouldn't be a lot of difficult cases. If so from where the experience? I would understand if they used some Gu to find or see the crime. But for now with just using intellect it looks forced and unrealistic. From clues there could be several possibilities and because of attack it should be impossible to investigate them. One possibility is some village found that mortal (slave/non clan member) cultivated and to allow him to join them and not kill his family is to kill genius from other village. If successful his family will be spared. If he also returns he can join shadow corps of the village....