The Ghost of Nasoj:
Although Nasoj's body was destroyed nearly 1300 years ago, his spirit remained trapped within the ruined foundations of his citadel, sealed within by the dark magic that filled the place. There, he gradually learned to maintain himself in energy-based form, as the gods and daedra had once done; and he learned to siphon off the magical energies deep within the earth as a power source. Feeding on this power, he grew stronger, and invisibly reached out to influence his followers to work towards his eventual release. When released, he will gain the power to possess the body of one of his followers -- an individual known in prophecy as the Vessel -- thus imbuing this person with frightening strength and other supernatural abilities. It is in this fashion that Kate will eventually confront Nasoj.
One possible way in which Nasoj could threaten the world again is if he manages to get his hands on the heart of Omega, the ancient automaton warrior. Omega's "heart" is the Spark of Life, an immensely powerful artifact of the extremely distant past. The Spark of Life will grant its wielder power on a level nearly equal to that of one of the gods before the Great Fall. This is enough power to make Nasoj the second most powerful being on Earth, after Kyia, and a potent threat to the civilized world. What's worse, the Spark is arcane power rather than divine power, and as such Merai would be unable to drain Nasoj of that power. Nasoj shouldn't actually get the Spark, at least not immediately, but it should be a definite goal of his cult's machinations. Even if he does eventually get the Spark, it will take time for him to figure out how to use it and access the full measure of its power -- time that the good guys can use to defeat him.