Before he left Metamor Keep on the mission to destroy Chateau Marzac -- a journey that would ultimately claim his life -- the seer Zhypar Habakkuk wrote down a final prophecy and delivered it to the care of Raven hin'Elric of the Lothanasi, with explicit instructions on when it was to be unsealed and read. After Habakkuk's death, the so-called Last Prophecy began to take on almost mythical overtones among those who were aware of its existence. It was, after all, not just the last prophecy of Habakkuk, but the last prophecy of the Felikaush, the bloodline of seers that began with Mad Felix of Lee. Habakkuk was the last of his kind, and had no children to carry on his legacy; with his death, the greatest line of prophets in history was extinguished.
Habakkuk's prophecy was opened, as instructed, on the day word came to Metamor that the Citadel of Nasoj had fallen to Keeper forces. The night before their assault, the Keepers had witnessed flashes of magic and the sounds of a great battle coming from inside the spire; when they went inside the next day, unopposed, they found the citadel in ruins and Nasoj's broken body among the rubble. The identity of the assailant was unknown, but speculation was rampant; the two prevailing theories were either that Nasoj's own lieutenants had betrayed him and fled, or that the Daedra Prince Ba'al had finally showed up to collect on the bargain the wizard had made with him so many years before. (In truth, it was neither; a Daedra Noble named Kagzenio, whom Ba'al had imprisoned thousands of years ago near the magical Nexus that powered Nasoj's citadel, had engineered an escape from his prison, swapping his own life force for Nasoj's and trapping the latter within the prison in his place.)
The Last Prophecy warned that Nasoj's spirit would arise again one day, more powerful and terrible than before. The Prophecy of Mad Felix, which had foretold Nasoj's doom, was still true, but its final fulfillment would come not in Thomas's own day but in the day of his distant descendants. Habakkuk foretold the coming of a person who would carry within him- or herself the blood of all of those great champions who had engineered the dark wizard's defeat. This person would become The Key, the central figure who would confront the risen Nasoj and ensure final victory.
While the Key's appearance was certain, however, the destiny of any single individual who might fulfill the requirements was far less clear. Habakkuk saw that another individual who shared this bloodline would become The Vessel, the one who would enable Nasoj's ascension by serving as a host for his now-imprisoned spirit. Just as Kagzenio swapped places with Nasoj, Nasoj would swap places with some other unfortunate soul. This was made possible because each potential Key carried the blood of Murikeer, who had been tied to Nasoj on a spiritual level since the battle in which they first encountered each other. Murikeer's blood would serve as a conduit for Nasoj to take possession of the Vessel, while the blood of the other heroes of Metamor would allow him access to a frightening array of magical abilities.
Both Kyia and her allies and the cultists of Nasoj have been actively tracking the appearance of potential Keys and Vessels, which have become more common in recent years as the numerous bloodlines involved have had more opportunities to intermingle. These individuals, whom the Immortals sometimes refer to as "pawns", are carefully monitored once Kyia and the others become aware of their existence, and may be contacted for more personal guidance and training if they show promising signs of being the actual Key. The cultists of Nasoj, on the other hand, have historically attempted to convert these individuals to their cause, killing them if they prove non-compliant. As they approach the fulfillment of their plans, however, and it becomes more feasible to actually free Nasoj from his prison, they have begun to kidnap potential Vessels and then hold on to them, in the hope that they will be able to take them to the ruins of Nasoj's citadel and perform the ritual that will bring about their master's ascension. (The ritual is a difficult one that must be performed at certain specified times, and only once a number of prerequisites have been met; if any of these conditions fail, the spell fizzles and the candidate dies. Thus, it behooves the cultists to have a healthy supply of potential Vessels to work with.)