Early game you need survivability,80 ap is not considered a low AP amount,if you farm nicely you can get out of the lane and have a rylai's before going for rabadons..if you don't then you don't farm properly,still,if you were reading my guide recently I wrote in Items'chapter that if you dominate a lane you can trade from rylai's to rabadon,but you rushed to downvote because I guess you have a karthus guide you want to upvote for yourself.

Next issue,skills,defile is a late game spell,early game you should focus on doing damage with Wall-Lay waste combo...you can sure try to deal with defile,but you will just try for quite a long time,MR reduction with mines is godlike early game,even at 2 lvl...if you prefer defile go ahead...but in most of the situations you will stay manaless early game with your mana pool...

Skills are your choice...go ahead and do whatever you want(I prefer both to ensure that I don't deal only lay waste damage)
Rylai's is to slow with defile to have better CC around you.

Now(AND the Archangel's Staff as the last item isn't good because there is no time to charge it.) Dude tear of the godess stack's it early and you use it late game...Learn before talking dude...just fail,you can pick the tear of the godess and you won't need clarity(which is for low skilled players,come on really?get clarity??get mana???lol).

Why to buy Rylai as the first expensive item? It gives you quite low AP and the slow you can ensure by your wall. Overmore, there is Flash missing there AND the Archangel's Staff as the last item isn't good because there is no time to charge it (in most games). The skill order is also quite strange - why don't max Lay Waste ASAP? And why to prefer Wall of Pain over Defile in the late game? There are many champions who hasn't as high MR. And bigger slow? You already have Rylai....

Thank you !

Pretty sexy guide you have there and yes i probulary wouldnt get CDR runes as his CDR is fairly low id stack more on the side of Ability power or something but over all good guide +1

Well is your call...I just consider CDR to be viable at any player,especially casters,you never know when you 'll need your ultimate and this runes make it ready faster...
Thanks anyway :)

Nice guide :) But i wouldnt get cd reduction runes.