Also, why did you put zhonyas and rylai's before deathcap? The early game ap from deathcap and tankiness from rylai's cannot be beat. Also, update to include individual items (ie. giants belt, blasting wand), and the order to build these in. Hope that helps.

Update the masteries when it is convenient.

Im still making it, its not complete yet. One of the Cons of this build is having low mana but it shouldnt be such a problem with Clarity but a Pro of it is having a Massive Damage to win teamfights even dead with not a high cost of mana.

Doesnt look too good...reasons why: Is a mejai's the smartest choice?Part of Karthus is getting deep in the teamfight instead of staying back like other AP carries so you would expect to die a bit

No mana items,dont you think you need at least some mana?Karth's pretty mana intensive...