Dreamthief hags steal dreaming minds in the Dimension of Dreams, trap them within dreamstones, and sell them to the highest bidder. It is difficult to determine exactly what harm befalls victims of dreamthief hags; they are never quite the same, and rarely ever sleep without encountering nightmares again.1
Dreamthief-born changelings, also known as waker mays, are among the rarest of all the hag heritages. Being outsiders, dreamthief hags cannot give birth to children of their own; instead, a waker may is created when a night hag devours the soul of an unborn daughter of a coven mate and replaces that soul with hers, killing herself to be reborn as a waker may and eventually become a dreamthief hag. Because the night hag dies in this process, she relies on her coven mates to follow her final commands to corrupt the newborn waker may into an eventual dreamthief hag.3
Dreamthief hags are almost never trusted, though many powerful evil beings who consider captured minds to be useful components in rituals or research welcome them as merchants. Their bite contains a venom capable of temporarily paralysing a victim's mind, and they have few scruples about capturing even long-time partners' minds if they feel offended.1
For additional as-yet unincorporated sources about this subject, see the Meta page.