Karthus the Mobile Dead Deathslinger!

I'm going to try and recreate this but basically I got piled on top of and nuked down with Karthus (surprise surprise) and at first couldn't cast any Qs (made me angry!) Then, suddenly, I'm able to move around while in my passive. The AoE around me followed and while I couldn't cast Q, I was able to move...

So I looked around and it sounds like the Q no casting is a known bug. That should be fixed but also if the moving passive works (even infrequently).