In some of the oldest records of the Inquisition that Diarmuid had encountered, or at least, the oldest ones pertaining to their actions during the founding of the Kingdom of Gaal, there were repeated mentions of 'hedge witches' and 'heretical, demonic shamen' who had to be purged from the land.
Before he'd come to the Vale of Mists, he'd accepted the teachings of the church that, even when a hedge witch offered healing, she did it for dark purposes that would only corrupt the minds and hearts of the people she turned into her unwitting thralls with her witchcraft.
