A merciless inquisitor operating in the Rhineland in the early thirteenth century, Conrad of Marburg's reports describing widespread Luciferan (that is, devil worshipping) heresy were accepted by Pope Gregory IX (ruled 1227-1241), becoming the basis for the pope's decretal letter Vox in Rama (A Voice in Rama), which described a heretical cult worshiping the Devil. This document was an important source for later ecclesiastical authorities seeking to link heresy with diabolism and devil worship, and can be seen as a precursor oflater descriptions of witches' Sabbats.