User blog comment:JBox2D/A dedicated anti-mage champion/@comment-4714821-20140223020109/@comment-4714821-20140225020813

You know, I never noticed this, but it seems like Karthus would completely counter her. Once she's burned her blink, if he gets up close with Defile active, he can get off his full combo regardless of the silences, since he can just sit next to her and she can't get away, and even when she does, he can just ult to kill her.

Basically, she can't block any of his abilities or stop him dishing out his damage, especially since Defile won't be blocked by her reduction. In fact, Ryze and most other mages that don't rely on skillshots would be able to do that provided they don't rely on massive burst like Veigar.