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Pro Karthus Build. (SHORT GUIDE)
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For harrassing try doing lay waste time to time, but dont get to close/vulnerable to be killed.
Use wall of pain when chasing or denying team intiations.
Always have Defile on when team battling. It does damage over time and can hurt people really badly with it on.
When you die you should use your ultimate, you may just get a kill or two.