Karthus is range mage dps champion. His skills let him slow down enemy, make aoe damage and kill fleeing enemies. Why it's my main champion? - because he can control game, and even with bad team mates it's possible to win. Karthus got one flaw: he is very weak in receiving damage so his health points bar can really fast disappear. But don't worry, I'm going to give you few tips, how you can remedy this.

First important thing you should know before start playing Karthus: its very difficult to play against , , .
There are also few items which can easy counter his ultimate: , ,
You think that is all? Not really, many champions got "counter Karthus ultimate" skills. Here is list (that's not all of them): , , , , , , , , , , , and many many more...

You should also know that all taunts (for example ) and stun (for example ) can interrupt casting your ultimate. If you die while you casting Requiem it wont work too. And there is one more thing: you must use ultimate in 4/5 sec of death (after this time, it deal 0 damage).

Karthus is mainly mid/solo top champion, but he can also go jungle or on lane with partner. You should remember that you are main carry in team, so invest in ability power items, some magic penetration and a bit in health/defence. In team fights stay back, put some lay waste near enemy, slow them by wall of pain and when they are close themselves turn on defile go as close them as its possible, press Zhonya's active skill on and deal huge damage. After that you probably will die, but don't worry, you are still in team fight so spam lay waste, and use Requiem. If you got time use wall of pain again. This is description of 90% fights with Karthus, he dies often, but he do really good job in team fights (dealing tons of damage).

At the top of this page, are 4 Karthus icons. Two of them are for jungle Karthus. First is for extreme dangerous and mega fun surprise jungling, which is damn difficult. Second is safe jungling, but as you know jungle Karthus is so easy to gank, that if you will give enemy any sign (for example smite icon on load screen), you can be sure that they will try kill you in jungle so often as its possible. And as you can expect, Karthus without blue buff, cant jungle!
Under Solo Mid inscription there are two different Karthus builds. Use first one if you play well and no need tons of mana, because your finger over "e" button is not very heavy, or you dont have adhd. Otherwise use second buld - mana based.

Main rules:
1. Don't start teamfights, you can't be first target!
2. If you missing mana in teamfights use clarity!
3. Don't use your ultimate, when there is nearby enemy who can interrupt it.
4. Before teamfight try to get blue buff.
5. Watch items which enemy has got, and calculate theirs magic resist and your magic damage. Don't waste Requiem!
6. If you know you are dyeing, try do it in best place (near teamfight area).
7. Check your defile toggle status. Don't waste mana on it, when there's no enemy in range.

You can check average Karthus stats in my ranked games here.