Build Guide by Drakmarine
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Not Updated For Current Season
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Karthus is a fragile character which is why the seals are so useful. This helps with survivability a surprising amount and for the more experienced Karthus player the health provides a nice competitive edge or can be swapped out for additional ability power/mana regen depending on your play style. The marks are pretty much unchangeable and the glyphs and quintessences are incredibly useful. Starting off with a mana crystal provides the ability to spam defile to good effect. In most instances the lay waste ability will be incredibly useful, especially in center lane, but in very rare circumstances it might be worthwhile taking defile first.
Early game can be a bit rough for Karthus, that is until he gets to be lvl 6. The best advice is to play on the conservative side and annoy the **** out of your opponents with lay waste. Defile as the second spell is a must and can easily make the difference between getting ganked and picking up first blood. I suggest laning solo with Karthus because reaching lvl 6 for him is a much bigger deal than for most champions. Picking up boots and the Tear of the Goddess while still in the early game is very doable and will keep you mana fed and safely speedy until you get your ult.
Once you get Requiem that is when Karthus's true middle game begins. Paying attention to all the lanes, especially in conflicts near towers, will pick you up a couple easy kills. Catalyst the Protecter is a decent mid game pick up item, but really its use is in getting Rod of Ages as soon as possible. Other mid game items after Archangels Staff are truly situational, but what I reccomend is above. Mainly in the mid game Karthus should be occupied with pushing lanes in team fights through appropriate use of defile and lay waste, but don't be afraid to sacrafice yourself to pull of a double or triple kill with your requiem. This will be more helpful then you realize come late game.
Late Game :
At this point you will have a decent ap base and the ability to last hit enemy champions constantly with your requiem. Depending on the situation you may want to jump for Zhonya's, but then again thats up to your disgression. In essence Karthus is going to be a little squishy, helped enormously by the seals, Rylai's and Rod of Ages. Even though you can do the damage of a carry if played correctly, problems can be caused by your genuine inability to take a hit and the limited ability of to cc of your wall. Just keep in mind your weaknesses and you will be able to dominate the late game period with your high damaging spells.
A very playable champion and incredibly fun he can take a little getting used to. I recommend playing multiple times with this build in order to get the full feeling of the champion, but you should be able to tell from game one that this build will support a competitive highly damaging Karthus capable of almost anything assuming your allies aren't terrible. If you have any questions, comments, or concerns please leave a comment and I'll try and address it. GL HF and play well.