what about the Seraph's Embrace in the first item?

Hey snu***z,
I think you've got a pretty neat guide on Karthus. :]
However, I think that you might want to add some visual tools to encourage the reader to stick with your guide. This guide might prove helpful along the way.
Also, I'd love to learn a lot more about your personal experiences, such as crazy to poor synergies since you only listed Jhin and Ashe so far but I'm sure there's many more add somewhere in the spectrum. Explaining your rune reccomendation would also be something that I would like to see. Like I get why you'd prefer Dark Harvest, but maybe not all of your reader will do and if they get it the can just skip the chapter. But the possibility that you're thinking of those who'd like to learn more from your knowledge, would surely benefit your guide.

Anyway, I think you're on a pretty nice track, keep it up!

Cheers, Wicked Cherry.

Humm... Jhin as ideal sinergy.....I like it!

Your guide is really good, also you explain very well why you think Karthus is a great support - the most important thing in making off-metas (besides actually playing) is defending your point of view.
However, you could add some BBCode to the guide, even the smallest highlight would do wonders to attract people to read your guide, you can refer to this guide for templates (or this wiki for a more precise explanation of functions).

Anyway, keep working on this guide, I really wish success to you! See ya!