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As Karthus play very defensive and try not to take damage as you are very weak at the start. Once you have Defile you need to concentrate on getting the minion kills as it will generate more mana for you to keep using your Lay waste ability. If you find yourself in trouble use the wall of pain and flash (only if you have to) to escape. Once you have the Rod of Ages you will become more durable so can start to push. In late game use your wall to get the magic resistance debuf and active defile to get a much higher DPS. Flash comes into its own hear as you can use it to keep the enemy in range of your defile. Also try to get your lay waste to only hit the enemy champion as if it only hits one enemy it will deal double damage. This means you my have to draw your opponents away from your lane to avoid hitting minions to gain maximum efficiency. Also don't forget that you get 7 seconds after death to try and get a kill. This is plenty of time to get a revenge kill or in late game deal extra damage to enemy champions in the big 5v5 battles.
Finally you need to use Requiem carefully. Work with your team to pick up kills from enemy champions that are recalling or retreating to gain health. Unless you are very good at timing your abilities don't bother trying to steal kills. You will most likely end up wasting Requiem and the long cool down will punish you. I have used Requiem to early many times and people have survived from its damage and as always the second after you use it the perfect opportunity arises and you inevitable miss a potential double kill. In late game when you start to organise full team pushes it is often a good idea to use Requiem before your team advance. It makes it a lot easier to take down the lower health champions if they start with a quarter of there health missing. Because of the 3 second cast delay you will often find it very hard to cast in the middle of battle so casting before is often your best bet and can easily turn a games result around as will often allow your team to pick of the high damage but low health champions of first leaving a relatively easy clean up of the rest.
Hope this helps people. This is my first build so please play before you rate and give feed back as any chance I can get to improve the build will be much appreciated. Please do not rate down due to lack of chapters. I plan to make this more comprehensive later when I have the time. (BUILD IS IN PROGRESS)