Knockdown, the status effect players don't even realize exist.
It cancels dashes like an interrupt cancels channels.
It's been in the game for quite some time, characters who have knockdown in their kit also have an additional CC effect tied to the knockdown effect to keep an enemy in that spot for awhile. Without knockdowns the dashes would completely move enemies to their ending point without stopping their movement.
Sadly riot decided to remove ahri's though :/
While grounded is a persistent effect of this and putting that on karthus who moves around constantly with it would be horrible. Unless you want it only when he dies then I guess that would be fine but constantly while moving is too strong.