I think the guide can be more informational, btw, I don't think Karthus need both Rod of Ages and Rylai's Crystal Scepter. In my opinion, Rylai's Crystal Scepter is better, because Karthus is actually not that mana-hungry, plus the runes you gave him, Rylai's is better.

For the Utility tree, you should split the 4 points in Swiftness into 3 in Meditation for extra mana regen. and 1 in Runic Affinity for longer blue buff duration.

Upvoted. Hope you're willing to take in my criticism.