@Riot How would Karthus be if we gave him the Grounded debuff on his W?

So I was just on the League Twitch today and I asked if they tried to switch out the slow on Karthus's W with the Grounded debuff that was given to Cass. While they said that they haven't tried it yet, one of them found it an interesting idea. So I thought I'd ask any of the other Reds and Community if Karthus would be better off with the Grounded debuff instead of either the slow or the MR shred.

Imo not only gameplay wise does it make sense, but Thematically as well. Gameplay wise, it gives him a reliable way to deal with Assassins that plague his Mid lane game so much, and if he goes Jungle he could use it to set up some good ganks. Thematically it makes sense because it's called wall of PAIN, not wall of Displeasure (despite what riot said when the wall got nerfed) it would be like his victims becoming wracked with pain so that they couldn't perform any extraneous movement (like, say DASHING).

Feedback is appreciated!