We've discussed this on the Ruin/Preservation thread.
Perhaps the Prophecy came from the Mist Spirit, I know it can't talk, but the main purpose of the prophecy was not to keep Ruin bound however, the main purpose of the prophecy was to find the person destined to defeat the deepness. I thinhk that there are two mist entities, The Deepness, which has allied itself with Ruin, and the Mist Spirit who has allied itself with Preservation.
I think it's safe to say that Ruin is the voice in Zane's head, because of how he keeps telling Zane to kill everyone, and how at the end, he tells Zane, "Of course I didn't tell you to kill HER!" because Vin is the one who goes on to free. Vin's mother on the other hand, who knows. Alendi also says he hears whispers that everything will be alright, and if he fails, another hero will arise to finish the job he started, these whispers, could bery well be Preservation.
I think the Mist Spirit is really an embodiment of balance between Ruin and Preservation, and the Deepness is the embodiment of unblalance, perhaps we see know deepness during the lord ruler's reign because he absorbed the deepness into himself, and then tipped the balance in Preservation's favour, preserving the world to the point of stagnation, thereby becomeing and embodiment of imbalance himself.