Karthus idea

I was reading a karthus post earlier and I thought one way to help karthus compensate for having 0 mobility in a game where probably over half the champions have tons of mobility or escapes is to add a grounding effect to his E ability and the longer the enemy stands inside of it the more it slows up to a total of 10% slow

I think the it will help him be sticky without the use of mobility plus it will actually punish enemies that dive into a karthus and dash around him dodging all his Qs