User blog comment:KaiserLos24/Mystery of the Shadow Isles (Summary)/@comment-3329904-20141101142009/@comment-12049719-20141102140027

Well, isn't Karthus a soul of the Shadow Isles? Similar to Hecarim and Mordekaiser? In the TOTBM cinematic, we see Hecarim coming out of the black mist in an attempt to harvest souls. Perhaps the same can be said for Karthus.

And he's called the death singer. A prophet. Prophets foretell what lies ahead and tell tales of the past, I believe. Just like in the bible, when prophets spread the words of ancient text. I strongly believe Karthus told us the tale of a spirit who seeks revenge, as evidenced by the lyrics of Lightbringer.