Why do demons seek to possess the living?
History claims they are malevolent spirits, the first children of the Maker, angry at their creator for turning from them and jealous of those creations he considered superior. They stare across the Veil at the living and do not understand what they see, yet they know they crave it. They desire life, they pull the living across the Veil when they sleep and prey on their psyche with nightmares. Whenever they can, they cross the Veil into our world to possess it outright.
We know that any demon will seek to possess a mage, and upon doing so will create an abomination. Most of the world does not know, however, that the strength of an abomination depends entirely on the power of the demon that possesses the mage. This is true, in fact, of all possessed creatures. One demon is not the same as any other.
Demons can, for instance, be classified. Enchanter Brahm's categorization of demons into that portion of the psyche they primarily prey upon has held since the Tower Age.
According to Brahm, the weakest and most common of demons are those of rage. They are the least intelligent and most prone to violent outbursts against the living. They expend their energies quickly, the most powerful of them exhibiting great strength and occasionally the ability to generate fire.
Next are the demons of hunger. In a living host they become cannibals and vampires, and within the dead they feed upon the living. Theirs are the powers of draining, both of life force and of mana.
Next are the demons of sloth, the first on Brahm's scale that are capable of true intelligence. In its true form, this demon is known as a Shade, a thing which is nearly indistinct and invisible, for such is sloth's nature. It hides and stalks, unaware, and when confronted, it sows fatigue and apathy.
Demons of desire are amongst the most powerful, and are the ones most likely to seek out the living and actively trick them into a deal. These demons will exploit anything that can be coveted—wealth, power, lust—and they will always end up getting far more than they give. A desire demon's province is that of illusions and mind control.
Strongest of all demons are those of pride. These are the most feared creatures to loose upon the world: Masters of magic and in possession of vast intellect, they are the true schemers. It is they who seek most strongly to possess mages, and will bring other demons across the Veil in numbers to achieve their own ends—although what that might be has never been discovered. A greater pride demon, brought across the Veil, would threaten the entire world.
--From The Maker's First Children, by Bader, Senior Enchanter of Ostwick, 8:12 Blessed.