Ithaeur are shamans of the People, gifted the barely visible face of Luna the patron goddess of mysticism. Those changed under the crescent moon share a mystical connection to the spirit world; it is their burden and duty to deal with denizens of Hisil.
Dealing with relatively friendy spirits is an easy to accomplish feat. But often it is the Spirit Master's role in a pack to wrestle the most malicious spirits into submission, and thus she might have the final word on dealing with issues in the spirit world. The Ithaeur also fits the role of the Ritual Master, allowing her to learn rituals notably easier than those who changed under a different auspice.