After thinking for a while, the old monk says: "The ritual of cleansing i can think of consists of two parts, which both must be conducted at the places from which the stones where taken: first the stone has to be cleansed of physical as well as spiritual taint by one knowledgeable in the ways of the spirits. Second the spirit of the holy place to which these stones are connected has to be appeased with an offering in order to undo the wrongs wrought by the servants of the Shadowlands. This offering is depending on the spirit, as far as i can say. some would probably be done with a few fruits and a prayer, but the older, more powerful one could demand something else entirely." He makes a pause, throwing a measuring glance at your captive. "Some require physical objects to change their behaviour, while others want for immaterial things like justice. Whatever you do with him lies in the hands of the Magistrate to decide. If he is willing to guide you to the shrines, so better be it, i think."