This slug like parasite is actually a symbiote. After attaching itself to a warmblooded host, it feeds off that being. However, its tenticles, growing under the skin, act as secondary nerves, increase the victims nerve responses.
The Hierophant of Greenmarch is a lycanthrope. Rather than seeking a cure, or hiding his condition, he considers it a blessing from the Goddess of the moon, and requires that all of the Druids and loyal Rangers of Greenmarch to share in his gift.