Pros

AOE
Powerful Late Game
Plenty of AP
Farmer

Cons

Squishy
Easy Target

I chose Teleport and Clarity for a couple reasons. Teleport helps you get to the action faster after dying. Also, in beginning game, since you're squishy, you go back often and teleport provides a much faster way of transportation.

I chose clarity because Karthus relies on his skills greatly. You would want to spam Lay Wastein the beginning of the match and would need lots of mana. Another option instead of clarity is flashto get away from being chased since Karthus is an easy kill.