Oneirogens were once normal mortals who suffered a rare but terrible curse. Within them lies a sliver-sized tear between planes, a crack hardly larger than a pinprick that destroys everything they once were in an endless deluge of planar power. Created by magical mistakes and otherworldly conjunctions, oneirogens exist in a haze as living husks whose grip on any reality has been erased. What little remains of their minds is bent on self-preservation, even as they are locked at the center of storms of strange powers endlessly streaming from their mouths and nostrils. The death of their personalities might be a mercy, but what lingers on are primal beings, slaves of uncontrolled planar might.
While the portal trapped within an oneirogen might connect to any plane of existence, it most commonly provides a path to the Dimension of Dreams. As mortal minds already drift toward that realm, breaches between the waking and unconscious worlds might occur when potent magics attempt to tamper with the boundaries of dream.