User blog comment:Eman4evr/Thinking of doing my provisionals. Advice?/@comment-10354824-20140227194933

I can give some tips about Karthus:

When NOT to pick Karthus

Well, Karthus isn't your best laner in the game, especially againts assasins. You CAN play againts LeBlanc(playing extremely aggressive after she uses W,denying bluebuff to her with the help from your jungler,she isnt a late game character, position carefully) and Kassadin(again, extreme aggression after he uses his Q, late game is harder though) IF they are not very good, but assasins like Talon(worst of them all), Zed(Zhonya and MAYBE you can win) and Kha'Zix(OMGKHAISBROKENFIXHIMFFS) are outright impossible to lane againts. Also watch out againts junglers with good gapclosers and CC (xin, for example)

When to pick Karthus:

When enemy midlaner picks a skillshot champion such as Lux( she outranges you, just farm and carry the lategame), Gragas(one of the easier matchups for Karthus, avoid the barrels),Victor(avoid the stun and LAZOR) and others that i cant think of right now.

%80 percent of the time, start with the boots so you can juke skillshots. Always watch out for easy kills with ult. Kill the melee minions with AA(ranged minions are too far away) to gain more mana. Buy a Rylai's Scepter(%35 slow if you it one target, kite like old blue ezreal, permaslow.) If enemy team doesnt build magic resist, buy Liandry's Torment. Aim for AT LEAST 100 farm at 15:00. Always flash INSIDE the enemy team if you can catch 4 people in your defile. Take Barrier, with the upgraded Archangel's Staff, you can get over  800 shield in teamfight, and with Zhonya(very important,use it ONYL when they all focus you) you can last for a long time. Always aim your Q to their ADC or APC(or fed jungler), Lay waste is very powerful againts single target, try to hit it only one target. With Rylai and good aiming, you can 3 shot their ADC before he/she can even AA you.

To sum it up, dont pick Karthus when enemy mid laner is LeBlanc, Talon, Zed, Kha'Zix, Veigar, Kassadin. Otherwise, feel free to carry the game with Karthus. And watch out for ganks. You can farm from 900 range easily, use this to your advantage. And ALWAYS try to AA last hit melee minions, Defile passive is important.