I was playing a game of ranked, I was playing a Soraka and our Sion in the middle lane killed the enemy Karthus, he went into ghost form then he will still able to move around. I saw him running around acting as usual while still in his passive after-death form.
I could not target him, I couldn't even click on him to see his mana. He had his e (defile) up at all times, never draining of mana. Not only I, but the Sion and Leesin in my game also experienced this issue, fortunately our Graves was able to autoattack him, I don't know why he was the only one able to attack him though.

This is a game breaking bug, we ended up winning the match because the enemy team had a leaver and we were in an advantage due to kills, but the Karthus could just ignore all of us minus Graves, and just spam spells.

Graves is really the best champ right now. So good at what he does, be that killing squirrels or immortal undead lich spectres.