An oneiromancer is a diviner who interprets dreams. The profession of oneiromancy seems particularly prone to the deceptions of charlatans though legitimate practitioners exist. It is unclear if it is necessary to be a mage for one to be capable of practicing oneiromancy.
In the "Side Effects" premium module of The Witcher computer game, it is mentioned that Deidre Ademeyn was the victim of a charlatan professing to be a mage specializing in oneiromancy. He conned his way into her bedroom and raped her. She is said to have exacted her revenge quite mercilessly.