There’s a chance Multiple Identity Disorder, aka Dissociative Identity Disorder, could be possession by a demon, spirit, or an inter-dimensional being(s) of some type. Although most of the time it’s a breakdown in the human brains interface with it’s own soul. The human soul is the culmination of a person’s parallel lives lived out in the various near infinite Parallel Universes encompassing almost every scenario within reason. A disorder in the mind can occur due to a combination of a genetic defect along with the deliberate, or accidental acquisition of heightened awareness within the higher dimensions without a strong will. The personalities of their parallel lives then leak in from the various parallel realities.
It seems most of the time a traumatic event triggers this mental defect among those with the genetic propensity for Multiple Identity Disorder, or what is sometimes called Split Personality Disorder. Despite this some cases really are just a person manufacturing new personalities to protect them from what they see as a traumatic world. Certainly our sub-conscious mind gives us a world of people we’ve created in our dream sleeping state. However the more advanced cases that have extremely intricate multiple personalities possessing knowledge, and skills the original personality never had are clearly parallel Universal in nature. That is why the paranormal community calls it Parallel Personality Disorder. In addition there is a relation to the paranormal power of Parallel Hypertelepathy. However in this case it is a wild untamed power the person has no control over.
The most extreme of these miraculous cases actually result in bodily transformations. Humans who are vampires, werewolves, and other supernatural beings in parallel Universes can sometimes acquire these parallel personalities in their minds. When these particular paranormal personalities take over their body we can sometimes see a temporarily transformation into said beings. One such case verified in a cryptozoological laboratory resulted in a person transforming into a powerful second generation vampire in dangerous situations. When it was over they were horrified by the carnage they created at they became disgusted by the blood dripping from their mouths. Their DNA reverting back to a complete human being without any trace of demonic based vampire DNA. This demonstrates the awesome power of our human minds! Certainly something seen with vampires experiencing amnesia.