Ult concept: Sludge river

I was thinking about the Aurelion Sol's Q and how it can grow infinitely and about anivias ult and how it grows for a few seconds in one location. Then i thought about an ult that sorta combined the effects. An ult that started as a small circle that is channeled and can grow infinitely. So i thought of an ult where the champion channels a sludge river that maybe flows in a target direction and just moves like a flash flood for a duration that varies depending on how long it is channeled. So if you only channel it for a few seconds it might be nearly useless, but if you channel it for a while, you will have this massive area of sludge that covers a massive portion of the map, slowing enemies inside, probably not dealing any damage though.

perhaps imagine a support that stands still in the back, channeling this massive AOE slow 10 seconds before a team fight even starts. It could give the allied team a significant advantage or just scare off the enemies as a large zoning method. It could be a crazy interesting ability