[GAMEPLAY] Fighting against Karthus and Kog'Maw and when they die they turn invisible.

So I was playing Veigar in an intermediate Co-op vs AI game and killed Karthus around 9 times and 3/9 times during his passive he would be invisible and cast his attacks. This also occurred with Kog'Maw 1/3 times, for example when I killed Kog'Maw you would see an invisible Kog'maw with his purple goop coming towards you.