Diablo is already fetching a high price, and that most likely is due to the hype it is getting from players
If, for one of the targets, you can't choose a new legal target, then it remains unchanged (even if the current target is illegal)
Each player who discarded a card this way draws a card
This can only be done during your turn, and only once per turn
As another example, if at the start of turn six you have five cards in your inkwell and hold a two-drop, a three-drop, and a five-drop, then just cast your five-drop and dont add anything else to the inkwell at this point
The rest have to be discarded, but it's a lot of aggression for a cost four card