The Oneiromancer is the spiritual head of the Moon Clan, who resides on Lunar Isle, near the astral altar. Like the rest of her clan, she is adept at magic and can use it to perform everyday tasks and float. As the head of the Moon Clan, she is in charge of the Waking Dream ceremony, in which a person enters their dreams so that they may become closer to magic and perform it at a level matching the Moon Clan.
When the player first meets her, she agrees to allow them to undergo the Waking Dream ceremony, so that a member of the Fremennik tribe (the player who became a Fremennik during The Fremennik Trials) may understand what it is to be a member of the Moon Clan and hopefully reduce the tensions between the two peoples. She helps the player by providing information on how to get hold of the Lunar equipment needed to complete the ritual, and will then brew the player a Waking Sleep Potion so that they may enter their dream and face the trials. Once the player completes the ceremony she will grant them permission to pray at the Astral altar to activate Lunar spells.
Her name is derived from oneiromancy which is a form of divination using one's dreams. Oneiro comes from the Greek noun ὄνειρος, meaning "dream". Combined with the suffix -mancy that is often used to refer to a type of magic in contemporary fantasy, the term could be used to simply mean "dream magician".
When you talk to her she says she can read your mind and when you say guess what I'm thinking she replies disgusted.
If you talk to her while your Lunar equipment is stored in your house, the player will try to get a second set from her, to which she will reply that she knows it is in there.