Serwe sees the halos both on Kellhus and the skin-spy Kellhus--perhaps we can infer that the halos are created by the watcher, cast onto the watched so to speak. Kellhus convinces Serwe he is divine and her belief is so poweful it begins to change him into divinity, an ascension that is ongoing through WLW. It might be that Kellhus is angling to produce a certain kind of belief at a very high level through: if the No-God returns, everyone will know that Kellhus was telling the truth about the coming doom.
Paradoxically, failing to prevent the No-God (and maybe getting killed in the process) might raise him to godhood. Would he have a blindspot for Mog? Or would he be the one god to reveal Mog to the others. Divine Kellhus leads Yatwer, Akkeagne, and Gilgaol to save the world!