"This one will not defy a Dreamer in his own domain. Their realm is beyond even that of Hell or Heaven. Here we may command them, but, break the link, and they may take what is theirs."— Xazax(src)
A Dreamer is a being from a realm beyond that of Heaven and Hell. They appear as small-toad-like, deformed creatures, far more so than any demon. They apparently spend their time in slumber. Forcing to wake from said slumber, and/or being on the mortal plane is tortorous for them.
Dreamers can be summoned into Sanctuary through magic. The spell requires the sacrifice of a soul. Wards must also be set in place so as to protect the casters from being overwhelmed by the forces of the spell. Dreamers themselves are very powerful, with neither the magic of Heaven or Hell able to affect them, other than drawing it away from its world and its slumber.