i Would suggest swapping 1 ring for Rylai. We know that he has wall for slowing but lay waste slow is still powerfull not mentioning some survivability with that HP boots. And you totaly lack survivability on this build.

And karthus is squishy true, but the rings on use effect is huge, and if u get rod of ages right off the bat, ull be fine. At the end of most games, full games, i usually have 2500 health, 2500 mana, and about 800 AP, assuming im at 20 stacks of soulstealer.

Not a fan of this build. 2 rings is bleh, the 25% ap bonus is unique, and the on use effect cant be used one after the other. Once you use one the other goes on cd as well. Ur alot better off getting two Archangel Staffs, being as the bonus 2.5% mana goes to ap does stack.