User blog comment:KaiserLos24/Mystery of the Shadow Isles (Summary)/@comment-3329904-20141101142009/@comment-3087008-20141102091222

It's probably reasonable to assume that Karthus is merely a servant of the Mist, delivering people into its embrace and harnessing the power of the tormented souls within it. For this entire time Karthus might have just been acting to prepare the world for when the Mist will swallow the world and bring about an age of darkness and death. Karthus refers to "his pilgrimmage"  and "You are only a herald, Hecarim. Remember who you truly serve." and finally "Countless voices in unison."

Our name is Legion, for we are Many