Talk:Karthus/@comment-5284325-20140604192704/@comment-15280970-20140606023005

his high bases are like that because karthus doesn't buy any ap early game (he has to rush ROA, tear and boots) and because higher ratios would make him broken later in the game.

Kassadin is the opposite case, he has low base to scaling ratio, to stop him from snowballing because he doesn't build ap early.

'''Flat pen isn't optimal for karthus. He has to get a lot of items before he can buy pen, and by the time he can void staff is more efficent. Explanation:'''

Karthus needs the greatest mana pool he can get, so he needs ROA and Archangels. The item that scales the most with those two is rabbadon's. Void staff is essential as always.

Then you have one extra slot. You can get penetration... or you can get Zhonya's.

Note: Remember that Ap increases your waveclear, penetration doesn't (minions don't have MR). Hence AP's rushing rabadon's first instead of void staff.