Dream Walkers

DnD: Kalashtar

While not technically a single distinct species, the phenomena is widespread enough to be categorized as such. Dream Walkers are mortals who have come to share their bodies with some extraplanar entity or spirit. This could happen through a multitude of possibilities: a mage projecting his consciousness into The Nihilint Expanse returning with a stowaway, a restless spirit being permitted to willfully possess someone, making a deal with some formless entity while dreaming, and so on.