Ok, guys, here we go. As you all know, and I thank RIOT for that, Karthus can be very imba, not only his Requiem, but everything else. How I see Karthus is that he has to do as much damage as possible. In this build there is no mana regen, but what could be done is taking the blue buff any time You can. Now lets talk trough. In early game, you buy some AP so you can do more dmg with your lay waste, the goal is to keep the enemy away while you kill as many creeps as you can, making sure that that number will be much higher than enemy champions in your lane. So in early game, keep enemies away with your Lay Waste, creep your way up to good items, and kill only if a good chance comes to do it. Mid game. Lets say you`ve come up to Rod of Ages. Now you are no longer so squishy, plus you can already do a lot of damage. Blue buff, blue buff, blue buff. Just a reminder :D. Now, at this stage, you can already do some decent one on one. But as the mid game goes, there are more and more 5v5 battles. So stick behind your tank, spam your Lay Waste, but focus squishy enemies, so you can get the their main dmg doers out of the way. Only if you are not focused, or most spells have been used up in the fight go into the crowd but still behind your tank with your defile on still spamming lay waste. This maximases the dmg you do. Requiem should be used at the end of the battle to finish off remaining enemies, but if the chance comes, well... :D. In the late game you should do pretty much what you did in the mid game. If you are runing from someone, you can always spam your Lay Waste and activate your Defile. You will do a lot of dmg to the chasing opponent plus you will heal yourself at all time with your spell vamp. Karthus is a great champion, and I hope you will like and enjoy this modest build. Have fun all :)