Karthus Q bug when passive activates

Well, recently I was playing Karthus, and when I died, I noticed a small visual and sound bug when dying When Karthus dies, he is immobile. But when I died this time, wherever I put my Q (Lay waste) down, it would not only go on it's max range, but it was also appearing wherever my cursor was with the visual FX and sound FX, but without the damage. I believe this needs to be revised, thanks for reading!