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I would suggest getting Zhonyas as about your 3rd item. Karthus is a walking target in team fights, he's slow, he has a huge defile affect to point him out and he has no escapes. Even when they know you'll be around for another 8 seconds, people will still try and focus you.
Zhonya's is a great counter to that, you keep your defile so you do some nice AOE damage over those 2 seconds and forces them to switch targets. When you're back from Zhonya's, you wont be focused right away. Assuming the fight stays in mostly one place, and assuming worst case you use Zhonya's 1 second in and you die 2 seconds after using it, you get 1+2+2+8 = 13 seconds of defile (and that's if you're really unlucky) plus your ultimate at the end. That combo usually amounts to around 2500 actual damage to _every_ hero around you, nevermind the lay waste damage.