The Prophecies is the shorthand term for the fate of the Phoenix Soul and the Soul Stealer, sometimes wrongly used as a term to refer to the two souls themselves.
While there are many suspected prophecies, the most common and widely accepted is that the Phoenix will continue to reincarnate until it can find a host that is capable of holding it. The Phoenix seeks the Soul Stealer, and conversely, the Soul Stealer is irrevocably drawn to the Phoenix.
The Phoenix plans to merge with the Soul Stealer, and once it has done so, will have found its perfect host and be able to begin its reign of destruction.
The Monte Cristos have been contracted to kill the Phoenix at each reincarnation, but also to try and figure out a way to stop its cycle, and to stop its reincarnation. The Seekers were formed to find out and kill each Soul Stealer, to prevent the Soul Stealers from wreaking general havoc with their powers. The League manages the two groups and makes sure everything is in order.
The Prophecies is also used as a term to refer to all the above groups at once.