Hello and welcome to my first guild.
So guys, I rarely see well played karthus's, as many lol gamers think karthus is just about his and that's what i thought first but then I wanted to try leveling up first, and the damge it can do to a single target is insane and the range is pretty good for a great harrasment at early game as it also has a short cd time.

Pros:
Karthus Karthus is a great mid owner
He's a great ap carry
can save his team mates with his [ because it has a high slow effect.
He has a skull head, what else do you want? duh!.. :P
low cd time on all of his ablities exept for his ulti
Cons:
He is so squishy u might wanna grab some armor if you're getting focused
He has the longest cd time ultimate in the whole history of LOL(I guess)

I'll try to put videos and visual teaching methods for you guys :) when I have the time.