Ok, so you just probably bought karthus right now and wondering.. how do i be pro and be a super annoying mofo? Well im here to help with my guide i usually do every game i play karthus. Its pretty good imho. At lvl 18 and the full build laid out without stacks, Is around 700 AP.

I get magic pen marks to do more damage, I get Mana Regen glyphs and seals for early game dominance with lay waste. However these mana regen runes are useless late game do to 3 archangels. So you may get other runes whichever you'd prefer.

I max out lay waste and defile ever. Lay waste first and Defile 2nd. Get wall of pain at lvl 4 for ganks and for escapes. And 1 point into your ultimate when availble.

You should always lay waste, lead up, (guess his movements) to hit the champion early game. Lay waste does double the damage when it only hits one thing. Either a creep or champion. Defile gives you mana whence killed a minion or champion. You should use this to your advantage and farm up well and easy. For your ultimate tell your teammates to tell you when to use your ultimate. However dont get greedy, if you know when your teammates are going to kill a champion don't KS because it may result in a waste of an ultimate.

FOCUS THE SQUISHIES/CARRIES!! This should be common sense

For harrassing try doing lay waste time to time, but dont get to close/vulnerable to be killed.

Use wall of pain when chasing or denying team intiations.

Always have Defile on when team battling. It does damage over time and can hurt people really badly with it on.

When you die you should use your ultimate, you may just get a kill or two.