AOSoL Lore: Cerebral Demons

History: Cerebral Demons, also known as Mind Demons, are mental creatures that have existed within Equestria since before it's founding.  They come into existence at a pony's birth and reside within their mind, laying dormant until they are awakened.  When awakened they become bound to whatever emotion it was that awakened them, and they strive to feed off of that emotion constantly.  They are not common creatures in Equestria, but its not unusual for there to be around half a dozen of them in existence at any point in time.  The process of how they come to exist in a pony is unknown, as its impossible to tell if one even is in a pony until it is awakened.  Unfortunately there is no way to predict whether a pony will be born with a Cerebral Demon or not, as there is no pattern based on the ponies they are found in.  

Dormant - Awakened: Cerebral demons lie dormant within a pony until they are "awakened".  This awakening occurs when their pony host has an extreme emotional experience.  It does not matter what emotion the host is experiencing, if it is strong enough, the demon will instinctively feed off of it, gaining power, strength, and awareness from it.  Once it has experienced that emotion, a cerebral demon becomes bound to that emotion for feeding.  It becomes its only goal in its existence, and it strives to make their host feel that emotion by any means necessary.

Image: Cerebral demons, upon awakening, take their form that will define their appearance for all of their existence.  Most often they will choose the same species as their host, sensing their inner magic upon awakening.  But some have been known to choose other forms than that of their host.  This can occur if there are other ponies or creatures near the host at the time of awakening.  They can take on the form of earth ponies, unicorns, pegasi, and, in a few cases, alicorns.  Their appearance varies greatly depending on which emotion it was that awakened them, with their color palettes ranging from dark colors for negative emotions, to bright colors for positive emotions.  They are naturally genderless, but their body types are normally quite tall, with heights averaging out somewhere between Princess Luna and Celestia.

Behavior: A cerebral demon is bound to their host's body and soul.  Because they are bound to them, they will often take on the traits and characteristics of their host, mimicking them and adapting it to their own personality based on their feeding emotion.  They are naturally genderless, but can take on male or female traits as they wish, often using them to help with feeding.  They are not naturally malicious, but they can be if they are feeding off of a negative emotion, or they can seem uncaring and insensitive if they feed off of a positive emotion.

Goals: Cerebral demons have one goal in their entire existence, to feed on the emotion that awakened them.  This goal is something they pursue with a burning passion. They will do anything to get their host to experience that emotion so that they can feed.  Their methods include mental invasion, coercion, manipulation, and even possession.  If a cerebral demon gains enough power from emotional feeding they can do one of two things, overwhelm their host's mind and take possession of their body, or summon themselves.  If they possess their host's body then the host's mind is forced into a wandering state, kept in a continuos loop of emotional experience by the shadow demon to provide a constant source of emotional sustenance.  Once in control the shadow demon will seek out and try to get other ponies to experience that same emotion so that they can feed off of them as well.  If they summon themselves, they are given a semi-real form and body that can interact with the physical world.  Here they can effectively influence their host to experience their feeding emotion. 

Feeding: When a cerebral demon is awakened they become dependent upon that emotion to feed themselves.  For their entire existence afterword they are only capable of feeling that emotion.  Further, they are completely incapable of feeling any other emotion, only the one which they feed on.  In order to feed a shadow demon only needs to experience that emotion, which can be done by reliving their host's memories, or tapping into their host's emotional state while they experience that certain emotion.  They can feed both actively or passively, focusing only on the feeding or accomplishing other tasks while they feed.  It is believed that the power they gain is simply the satisfaction of being able to feel any emotion at all.  Theories suggest that when they are not feeding shadow demons feel nothing, only emptiness.  Their drive to feed comes from the desire to feel any emotion at all.

Intelligence: A demon's intelligence, awareness, and general knowledge usually stems from their host's own knowledge.  They can also gain information and even learn new things on their own but their understanding is usually limited to what their host's know.  However, there have been a few cases where shadow demons showcased exceptional intelligence, even when their hosts were nowhere near that level.  These demons were the most dangerous, as they could easily outsmart their hosts, manipulating their emotional state, making them their puppets, or even just taking their bodies permanently as their own.  Unfortunately, these hyper intelligent demons were too smart to be held back by enchanted items alone; they required their hosts to also be mentally capable of defending themselves, but most were incapable of doing so.  

Defense: Early in Equestria's history, one of the biggest worries were the cerebral demons, as they could essentially take over a pony if they fed on that pony's emotions for long enough.  They were impossible to destroy completely as doing so requires the destruction of the host themselves, which ponies were unwilling to do.  The best solution was purposed by Starswirl the Bearded in conjunction with Princess Celestia and Princess Luna.  The solution was to create normal clothing items that were enchanted with strong spells designed to keep the cerebral demons from possessing a pony's body, in addition to strengthening their mind.  These items proved to be incredibly effective, as they could completely incapacitate a normal cerebral demon, preventing them from ever harming their host.  However they were less effective against hyper-intelligent demons, who would simple manipulate their hosts into removing the artifacts and then destroy them with their host's body.  If a hyper intelligent cerebral demon releases itself upon Equestria, it seeks out ponies it can feed off of, and will often break those ponies' minds in the process of forcing them to experience that emotion.  These demons become an extreme danger to Equestrian citizens, and are impossible to control, and so the only solution at that point is to destroy them and their host.

Background canvas taken from 

NOTE: I changed the name of the species from "Shadow" to "Cerebral/mind" demons, as the term "shadow" has been used so much to describe dark and evil forces in Equestria.  Plus, cerebral is more nuetral, which more correctly applies to less dangerous demons.