I found an insanely op karthus build it makes his ulti drop over 3k.

I chose all mana per level runes because you need the mana for the arhangels

Standard caster

You want 5 archangels for the passive with mana giving ap and rabadons for increased ap

You want this skill sequence lol its good

You want ignite for damage and the other summoner spell doesn't matter

You start off with a person helping you to get blue buff then solo mid and harass there mid with your q spell and tell tour team when you have your ulti when you get it so they can tell you when to use it