You know, I really like the concept here. It isn't wildly different from your standard demonic pact, but I don't really mind. I like how you took a relatively simple idea (sorcerous magic comes from a possessing spirit) and really fleshed it out to fit within your world. Well done, and thanks for the enjoyable read.
Ultimate power, a bit of madness, the risk of dying horribly... what's not to love about this.
I like the flavor that bleeds through. These aren't normal sorcerers who gather power through bound spirits. There is an entire ecosystem of ethereal creatures. It speaks to a larger, well-developed world, with enough similarities to standard fantasy to make it easy to grasp and enough uniqueness to set it apart.
Well, I was thinking of calling them something different. I chose to stick with Sorcerer simply because it was something people already knew, and it wouldn't be confused with the distinctly different "Wizard." But "Sorcerer" is simply the terminology that the Secta Arbite denotes to them. I'm sure the fearful populace has a number of colorful names to cakk them.
And to be honest PoisonAlchemist, I'm not entirely certain what happens when they die. I would assume that since the Prophet draws it's life force from the soul of it's host, when that soul vacates it can no longer sustain itself. However it is also bound at a physical and spiritual level as well, so the trauma of a "Real" death might simply kill the prophet as well.