The way I look at it, the best analogue to the Fade is the Warp in the Warhammer 40K universe - a parallel, immaterial dimension where the thoughts and feelings of mortals can leave imprints (and also the source of all magic). The Fade itself would be a neutral realm, but whenever we sleep our consciousness fills this dimension with memories, virtues, vices, ideologies, emotions, fears, hopes etc. If, let's assume, similar thoughts and emotions attract each other, then the more mortals experience a single emotion or a single concept, the more it accumulates into the Fade, until it creates a "sentient" being which completely manifests said concept/emotion/virtue/vice. So, either WE create spirits and demons simply by virtue of unconsciously influencing the Fade, or these creatures already exist in a neutral state and they just "pick" and embody a specific concept we present them with. Either way, we create the Fade, the spirits, demons, and everything inbetween. The assumption that the Fade is primarily demon-infested would mean Thedas as a whole is full of vulgar and corrupt mortals, or simply that there are too many bad things happening to people, which trigger bad emotions, like a vicious cycle. Interestingly, if we go with the ideas of us creating Spirits and Demons, I would assume mortals could easily create Gods if billions of people belief in the exact same imaginary deity, and that idea is reflected in the Fade and is thus given life. Maybe that's how the Elven Gods came to be. Just food for thought 