Talk:Karthus/@comment-26489540-20151206133502/@comment-24385282-20151216000438

You would need at least 750 AP for his ult to be dealing 1k damage (even then, that's without calculating enemy MR), I wouldn't say it's worth getting that much AP (you'd need 5 high-AP items with little room for defense or utility options) just to do a bunch of damage on a spell with that long of CD. You generally don't need much AP for your ult to do enough damage, it's already global and auto-targets every enemy champion and still has decently high damage. Besides, it's not really viable in the current meta to base anything off "endgame." It's not impossible to reach it but it's arguably harder than ever or at least close to.

If anything I would consider grabbing a Rylai's for the insane utility it brings to a champion like Karthus - the range and spam of Lay Waste, plus the constant damaging aura means extremely powerful slows, plus the item gives a decent amount of both health and AP. Even your ultimate can allow you to give some across-the-map CC (40% for 1.5 seconds, I think. Not sure what Karthus' ult is considered in terms of slow type for Rylai's). If you do purchase this, Liandry's would definitely be a powerful replacement for another one of the items you've listed, most likely Abyssal.

Seraph's-RoA is a powerful item combo in terms of damage, defense, and utility, especially on Karthus  (much like Liandry's-Rylai's), but I wouldn't pick it up if you don't intend to grab a Rabadon's as well. So, in conclusion, with the build you specifically mentioned, I would definitely round it off with a Deathcap, but I personally think trying out other sets and seeing how they work for you would be a good idea as well.